Thyroid hemiagenesis is a rare condition present from birth where one side of the thyroid gland does not develop. Many people with thyroid hemiagenesis have no symptoms and feel well, so it is often found by chance in childhood or adulthood during a scan or checkup. Not everyone will have the same experience. Some people with thyroid hemiagenesis have signs of an underactive or overactive thyroid, such as tiredness, weight change, feeling unusually cold or warm, or a neck lump from disease in the remaining thyroid tissue. Thyroid hemiagenesis itself is not usually life-threatening, and treatment is only needed if thyroid hormone levels are abnormal or another thyroid problem is present, which may include monitoring, thyroid hormone medicine, or treatment for overactive thyroid disease.
How many people live with Thyroid hemiagenesis?
There is no single official worldwide or U.S. count for thyroid hemiagenesis, where one side of the thyroid never develops, but widely cited studies suggest it is present in about 0.02% to 0.2% of people, with the range changing depending on how people are screened and diagnosed (Ruchala et al., 2010). Using that range, this suggests roughly 1.6 million to 16 million people worldwide (Ruchala et al., 2010; United Nations, 2024) and about 67,000 to 670,000 people in the United States (Ruchala et al., 2010; U.S. Census Bureau, 2024).
Overview
Thyroid hemiagenesisSymptoms
Thyroid hemiagenesis often causes no noticeable signs and may be found by chance on imaging. If thyroid hormone levels are low or high, people with thyroid hemiagenesis can develop tiredness, weight changes, neck swelling, or a fast heartbeat.
Outlook and prognosis
Many people with thyroid hemiagenesis stay well for years, especially if the existing thyroid lobe makes enough hormone. Outlook is usually good, but some develop hypothyroidism, nodules, or an enlarged thyroid, so regular thyroid blood tests and checkups matter.
Causes and risk factors
Thyroid hemiagenesis usually develops before birth, often from a chance change in early thyroid formation. In some families, thyroid hemiagenesis may relate to inherited genetic factors, while sex, other thyroid differences, and autoimmune thyroid disease can affect whether problems develop later.
Genetic influences
Genetics may play a role in Thyroid hemiagenesis, but in many people no clear genetic cause is found. Rare familial cases suggest inherited factors can matter, likely alongside early developmental events before birth.
Diagnosis
Diagnosis of thyroid hemiagenesis usually starts when imaging shows only one thyroid lobe. Doctors confirm it with ultrasound, and sometimes a thyroid scan or other imaging to rule out surgery, shrinkage, or another thyroid problem.
Treatment and drugs
Treatment for thyroid hemiagenesis depends on how well the remaining thyroid works. Many people with thyroid hemiagenesis only need regular thyroid blood tests and checkups. If hormone levels are low, daily levothyroxine can replace what the thyroid is not making.
Symptoms
Many people with Thyroid hemiagenesis feel completely well and only learn about it after an ultrasound or another scan for an unrelated reason. In this condition, one side of the thyroid did not form before birth, while the other side may still make enough hormone for the body. The early features of Thyroid hemiagenesis are often subtle or absent. A healthcare professional can help sort out which changes are linked to thyroid hormone levels and which may have another cause.
Many people with Thyroid hemiagenesis have no obvious day-to-day signs. The condition is often found by chance during an ultrasound or another neck scan.
Some may notice fullness low in the front of the neck or a small lump. This usually happens when the remaining thyroid grows larger or develops a nodule.
If the remaining thyroid does not make enough hormone, tiredness, feeling cold, constipation, dry skin, and weight gain can appear. Many living with Thyroid hemiagenesis then find everyday tasks take more energy than usual.
In some people, the remaining thyroid becomes overactive, leading to shakiness, sweating, a fast heartbeat, trouble sleeping, or unplanned weight loss. Day to day, it can feel like your body is running too fast even when you are resting.
How people usually first notice Thyroid hemiagenesis?
Thyroid hemiagenesis is usually first noticed when a child, teen, or adult has a neck ultrasound for a thyroid lump, abnormal thyroid blood test, or symptoms such as tiredness, trouble with growth, palpitations, or a visible neck swelling, and doctors realize that one side of the thyroid never formed. In many living with this condition, there are no obvious first signs of thyroid hemiagenesis at all, and it is found by chance during imaging or an exam for another thyroid problem. Less often, how thyroid hemiagenesis is first noticed is through related thyroid disease in the remaining lobe, such as hypothyroidism, overactive thyroid, or a goiter, which is an enlarged thyroid.
Types of Thyroid hemiagenesis
Thyroid hemiagenesis is a birth difference where one side of the thyroid gland does not develop. The main variants of thyroid hemiagenesis are based on which thyroid lobe is missing, and in some people the thin bridge between the two sides is also absent. Symptoms don’t always look the same for everyone. Many people have no symptoms at all, while others are found to have a neck lump, an overactive or underactive thyroid, or changes picked up during imaging for another reason.
Left lobe absent
This is the most common type of thyroid hemiagenesis, with the left side of the thyroid missing and the right side present. Many living with this type feel completely well, but some may develop thyroid hormone imbalance or enlargement of the remaining lobe. When symptoms do happen, they are usually related to how well the remaining thyroid tissue works rather than the missing side itself.
Right lobe absent
In this rarer type, the right side of the thyroid does not form and the left side remains. Day to day, symptoms are often similar to the left-sided form, and many people only learn about it after a scan or checkup. Some may notice fullness in the neck or symptoms of an underactive or overactive thyroid if the remaining lobe is affected.
Absent isthmus
Some types of thyroid hemiagenesis also include a missing isthmus, the small strip of thyroid tissue that normally connects the two lobes. This usually does not cause separate symptoms on its own, but it can change how the gland looks on imaging or during surgery. For many, any symptoms still depend mainly on the health and size of the remaining thyroid tissue.
Can I slow down the onset of symptoms?
No, there is no good evidence that lifestyle changes or medicine can meaningfully delay the start of symptoms in thyroid hemiagenesis, because this is a difference in thyroid development that is present from birth, and many people never have symptoms at all. Healthy habits are still good for your overall health, but they have not been shown to prevent thyroid problems from starting; the most helpful step is regular checkups, often with thyroid blood tests, so an underactive or overactive thyroid can be found and treated early if it happens. That cannot prevent the condition, but it can help many living with thyroid hemiagenesis stay well and avoid untreated problems.
What genetics can tell you about symptoms
Genetics may help explain why thyroid hemiagenesis happens in some families, but for most people it does not clearly predict which symptoms they will have day to day. Many living with this condition have no symptoms at all, while others develop an underactive or overactive thyroid, and inherited differences may play a part in that risk, though the link is not strong enough to forecast exactly how severe symptoms will be or when they will start. In other words, genetic testing can sometimes add context, but it usually cannot tell for certain whether someone will feel well, develop hormone-related symptoms, or need treatment later on.
Causes and risk factors
Thyroid hemiagenesis usually starts before birth, when one side of the thyroid does not fully form. In many people, the exact cause is unknown, but inherited or new genetic changes in genes that guide thyroid development may play a role. Researchers are still learning how these pieces fit together. The known risk factors for thyroid hemiagenesis are limited, though it is reported more often in females and sometimes runs in families. Lifestyle habits do not seem to cause thyroid hemiagenesis, but iodine levels and other thyroid conditions can affect how well the remaining lobe works later in life.
What are the most common causes or risk factors?
Thyroid hemiagenesis means being born with only one half of the thyroid gland, and it usually happens because one side does not fully develop very early in pregnancy for reasons that are still not clear. Most cases seem to happen by chance, but rare family cases suggest that inherited gene changes can play a role in some people. There are no well-proven lifestyle or environmental risk factors, and it is not thought to be caused by anything a parent did during pregnancy.
Environmental and biological risk factors
Thyroid hemiagenesis is a birth difference in which one side of the thyroid does not develop. When people look into the causes of thyroid hemiagenesis, the honest answer is that doctors still cannot identify one clear reason in most cases. Doctors often group risks into internal (biological) and external (environmental). For this condition, only a few biological patterns are known, and no outside exposure has been firmly confirmed as a cause.
Most cases happen without an obvious trigger that doctors can identify. This means thyroid hemiagenesis is usually not linked to anything clearly done or not done during pregnancy.
Thyroid hemiagenesis is reported more often in females than in males. This is one of the most consistent biological patterns seen, although it does not explain the condition by itself.
The condition starts very early before birth, when one thyroid lobe does not form as expected. This points to a change in fetal development rather than a condition that develops later in life.
No medication, infection, chemical, or other environmental exposure has been firmly proven to cause thyroid hemiagenesis. Researchers continue to study pregnancy-related influences, but clear environmental risk factors have not been established.
Genetic risk factors
Many living with thyroid hemiagenesis never learn why one side of the thyroid did not develop. Most cases seem to happen sporadically, with no single inherited cause identified. Still, thyroid hemiagenesis can run in families, and rare gene changes involved in early thyroid development have been reported. This means the genetic causes of thyroid hemiagenesis are likely mixed and not yet fully understood.
Family history: Having a close relative with thyroid hemiagenesis may raise the chance of the same thyroid difference. This does not mean it will be passed on in a simple way, but it supports an inherited contribution in some families.
Development genes: Rare changes in genes that guide early thyroid formation have been reported in some people with thyroid hemiagenesis. PAX8 is the best known example, but no single gene explains most cases.
Related thyroid defects: A family history of other thyroid development problems, such as congenital hypothyroidism caused by an underdeveloped or misplaced thyroid, may point to shared genes. These conditions can cluster in families even when relatives do not have the exact same thyroid difference.
Syndromic conditions: In a small number of cases, thyroid hemiagenesis appears as part of a broader inherited condition that affects development in more than one body system. When this happens, genetics may be easier to identify because the thyroid finding is one piece of a larger pattern.
Lifestyle risk factors
Thyroid hemiagenesis is present from birth, so lifestyle habits do not cause it. What matters is how lifestyle affects thyroid hemiagenesis by changing how well the remaining thyroid lobe can meet the body’s hormone needs and how symptoms are managed. The biggest effects usually involve iodine exposure, eating patterns, smoking, exercise balance, sleep and stress, and consistent treatment habits when thyroid hormone is needed.
Too little iodine can make the single thyroid lobe work harder and may increase the chance of enlargement or underactive thyroid function. Very high iodine intake from supplements, kelp, or iodine-rich products can also disrupt hormone production in some people.
Thyroid support products, seaweed tablets, and high-dose biotin can complicate monitoring by affecting thyroid function or making blood test results harder to interpret. People taking levothyroxine can also absorb it less well if they use calcium, iron, or fiber supplements at the wrong time.
Very restrictive dieting or poor overall nutrition can worsen fatigue, muscle weakness, and other symptoms if thyroid hormone levels are already borderline. Adequate protein, iron, selenium, and zinc intake helps support normal hormone production and conversion in the body.
Smoking can interfere with thyroid regulation and may increase the risk of thyroid enlargement or abnormal thyroid function over time. In someone with only one thyroid lobe, that added strain may matter more because there is less reserve tissue.
Risk prevention
Thyroid hemiagenesis means one side of the thyroid never fully develops before birth. There is no proven way to prevent thyroid hemiagenesis itself, because it is usually a congenital difference rather than something caused by day-to-day habits. Prevention instead focuses on lowering the chance of problems linked to it, such as an underactive or overactive thyroid, thyroid nodules, or delayed recognition of early symptoms of thyroid hemiagenesis. Prevention works best when combined with regular check-ups.
Routine medical visits can help catch thyroid hormone changes early, even if you feel well. This matters because many people with thyroid hemiagenesis have no symptoms at first but can later develop thyroid imbalance.
Periodic blood tests can show whether the remaining thyroid lobe is making too little or too much hormone. Checking levels over time helps guide treatment before fatigue, weight change, palpitations, or other symptoms become more disruptive.
A thyroid ultrasound can confirm the thyroid structure and look for nodules or enlargement in the existing lobe. Follow-up imaging may be advised if an exam changes or if new neck swelling appears.
Getting a normal amount of iodine supports healthy thyroid function, but too little or too much can cause problems. Use supplements only if a clinician recommends them, since extra iodine is not a routine way to prevent thyroid hemiagenesis complications.
Can you prevent Thyroid hemiagenesis?
Probably not. Thyroid hemiagenesis means one side of the thyroid does not fully develop before birth, and there is no proven way to prevent it because the exact cause is usually not clear. Good pregnancy care, including avoiding smoking and keeping regular checkups, is important for overall health, but it has not been shown to prevent this specific condition.
Transmission
Thyroid hemiagenesis is not contagious, so it cannot be passed from one person to another through touch, coughing, sex, food, or blood. It is a birth condition that develops before birth, when one side of the thyroid does not form.
For many living with thyroid hemiagenesis, the main question is how Thyroid hemiagenesis is inherited. In most cases, there is no clear family pattern and it seems to happen sporadically, although rare family clusters suggest genes may sometimes play a part. This means the genetic transmission of Thyroid hemiagenesis is not fully understood: it may occasionally run in families, but many cases likely result from a new developmental change rather than something directly passed from a parent.
When to test your genes
Genetic testing is not routine for thyroid hemiagenesis, because most cases happen sporadically and results rarely change day-to-day care. It may be worth discussing if you or your child also have other birth differences, thyroid problems running in the family, or features suggesting a broader genetic syndrome.
Diagnosis
Many people with thyroid hemiagenesis do not know it is there until a scan is done for a thyroid lump, abnormal thyroid blood work, or neck swelling. Sometimes only mild symptoms show up, such as tiredness, a sense of fullness in the front of the neck, or no symptoms at all. For some, routine check-ups reveal the first clues. The diagnosis of thyroid hemiagenesis is usually made by finding that one side of the thyroid gland did not develop and then checking how well the remaining thyroid tissue is working.
A clinician may feel the front of the neck to check for thyroid enlargement, a lump, or asymmetry. This exam cannot confirm thyroid hemiagenesis on its own, but it can help guide the next tests.
Ultrasound is often the main test because it can show whether one thyroid lobe is absent and whether the remaining lobe is enlarged or has nodules. It is painless, does not use radiation, and is central to the genetic diagnosis of thyroid hemiagenesis only when imaging findings are paired with the overall clinical picture.
Blood tests usually measure thyroid-stimulating hormone and thyroid hormones to see whether the remaining thyroid tissue is keeping hormone levels normal. These tests also help detect an underactive or overactive thyroid, which can occur in some people with thyroid hemiagenesis.
A thyroid uptake scan may be used when ultrasound findings are unclear or when doctors need to see how active the thyroid tissue is. This can help distinguish a truly absent lobe from thyroid tissue that is present but not functioning well.
How is Thyroid hemiagenesis diagnosed?
Thyroid hemiagenesis is usually diagnosed with an ultrasound of the neck, which shows that one side of the thyroid gland is missing. Doctors often also do blood tests to check how well the thyroid is working, and they may use a special thyroid scan to confirm that the missing side truly never formed and is not just hidden or not active. Genetic testing is not usually needed, because the diagnosis is mainly made from the imaging results and thyroid blood tests.
Stages of Thyroid hemiagenesis
Thyroid hemiagenesis does not have defined progression stages. This is because it is a difference in how the thyroid gland formed before birth, rather than an illness that usually moves through early, middle, and late phases over time. Often the process begins when you notice changes in daily life, such as tiredness, a neck lump, or abnormal thyroid blood test results, even though many people with thyroid hemiagenesis have no symptoms at all. The diagnosis of thyroid hemiagenesis is usually confirmed with an ultrasound scan, sometimes along with a thyroid scan and blood tests, and follow-up mainly checks whether the remaining thyroid tissue is working normally.
Outlook and prognosis
The outlook for people with thyroid hemiagenesis is generally excellent, especially when the remaining thyroid lobe produces enough hormone to maintain normal thyroid function. Many individuals remain asymptomatic for life and are diagnosed incidentally, but some develop hypothyroidism, hyperthyroidism, thyroid nodules, or autoimmune thyroid disease, which can affect long-term management rather than overall survival. Prognosis is usually determined by these associated thyroid conditions rather than by the anatomic absence of one lobe itself. With regular follow-up and treatment when needed, most people can expect a normal life expectancy and good quality of life. Thyroid hemiagenesis is not generally considered a life-threatening condition, and mortality is not increased unless a separate serious thyroid disorder, such as malignancy, is present.
Long-term effects
Many people with thyroid hemiagenesis live full lives and may not notice any day-to-day effects for years. The long-term picture is often shaped by how well the remaining thyroid tissue keeps up with the body’s hormone needs. In some people, thyroid hemiagenesis stays stable, while others develop changes such as low thyroid hormone levels, enlargement of the remaining lobe, or thyroid nodules over time. Long-term effects vary widely, and some changes are found only on blood tests or imaging.
Many people with thyroid hemiagenesis keep normal thyroid hormone levels for years or lifelong. In these cases, energy, growth, weight, and body functions may remain typical despite having only one thyroid lobe.
Some people eventually develop an underactive thyroid, which can lead to tiredness, feeling cold, dry skin, constipation, or slower growth in children. This is one of the more important long-term effects of thyroid hemiagenesis because it may appear later, even if early symptoms of thyroid hemiagenesis were mild or absent.
The thyroid tissue that is present may enlarge over time as it works harder to meet hormone needs. This can sometimes cause a visible neck fullness or a goiter, especially if hormone demand increases.
People with thyroid hemiagenesis may be more likely to develop lumps in the remaining thyroid lobe. Most nodules are benign, but they can still lead to more testing and, less often, treatment if they affect hormone levels or raise concern on imaging.
How is it to live with Thyroid hemiagenesis?
Many people with thyroid hemiagenesis, meaning they were born with only one thyroid lobe instead of two, feel completely well and may not learn about it until an ultrasound is done for another reason. Day to day, life is often normal, but some living with thyroid hemiagenesis need periodic blood tests and checkups because the single lobe can sometimes keep up easily and sometimes becomes underactive, overactive, or develops nodules; if hormone levels drop, a daily thyroid hormone tablet can usually keep energy, mood, weight, and body temperature on track. For family and others around you, the effect is usually small, though they may notice the extra medical follow-up or help with appointments, and reassurance matters because this finding often sounds more alarming than it turns out to be.
Treatment and drugs
Thyroid hemiagenesis means one half of the thyroid did not develop, and treatment for thyroid hemiagenesis depends less on the missing lobe itself and more on how well the remaining thyroid tissue is working. Many people with thyroid hemiagenesis do not need medicine at all if their thyroid hormone levels are normal and they feel well, but regular checkups and blood tests are often advised. If the thyroid is underactive, treatment usually involves daily thyroid hormone replacement, most often levothyroxine, to bring hormone levels back into a healthy range. If there is a goiter, overactive thyroid, inflammation, or a thyroid nodule in the remaining lobe, doctors treat those problems in the usual way, which may include medication and, less often, surgery. Not every treatment works the same way for every person, so follow-up with an endocrinologist can help match care to your symptoms, lab results, and ultrasound findings.
Can you cure Thyroid hemiagenesis?
No, thyroid hemiagenesis cannot be cured because one side of the thyroid did not develop before birth. Many people with this condition do not need treatment if the remaining side makes enough thyroid hormone, but regular checkups are often important. If hormone levels are low or another thyroid problem develops, treatment can replace the hormone and help manage symptoms, but it cannot grow back the missing side.
Non-drug treatment
For many living with thyroid hemiagenesis, day-to-day care is less about active therapy and more about checking that the remaining thyroid tissue is doing its job well. Many people wonder about treatment for thyroid hemiagenesis when thyroid levels are normal, and in that situation careful follow-up is often the main approach. Not every approach works the same way, because care depends on whether thyroid hormone levels stay normal and whether there are nodules, swelling, or pressure symptoms in the neck.
Scheduled visits with an endocrinology specialist help track how well the single thyroid lobe is working over time. This can catch early changes before symptoms become obvious.
Thyroid blood tests are used to look for signs that the gland is underactive or overactive. These checks are especially helpful if you notice tiredness, weight change, palpitations, or other early symptoms of thyroid hemiagenesis-related hormone imbalance.
Ultrasound can confirm the thyroid shape and monitor the remaining lobe for nodules or enlargement. It is painless and can be repeated over time without radiation exposure.
Paying attention to changes such as neck fullness, trouble swallowing, hoarseness, or a new visible lump can help guide when reassessment is needed. Keep track of how lifestyle changes affect your symptoms if another thyroid condition is also present.
Your genes may have an impact on how drugs work for you
For most people with thyroid hemiagenesis, gene differences do not usually play a clear, everyday role in deciding treatment. If the remaining thyroid does not make enough hormone, many living with this condition take thyroid hormone medicine, and the dose is usually adjusted based on blood tests and how they feel, not on genetic testing. In other words, medicine response can vary from person to person, but for this condition, careful follow-up matters much more than looking for inherited differences.
Pharmacological treatments
Many people with thyroid hemiagenesis do not need medicine at all, especially if the remaining thyroid lobe makes enough hormone for the body’s needs. Treatment depends on what the single thyroid lobe is doing, so care is usually aimed at hormone imbalance rather than the missing lobe itself. Medication can feel like a big step, but the most common drug is thyroid hormone replacement when thyroid levels are low. In a smaller number of cases, medicines may also be used if the remaining thyroid becomes overactive.
Levothyroxine is the main drug used when thyroid hemiagenesis leads to an underactive thyroid, also called hypothyroidism. It replaces the hormone your thyroid would normally make and can help with tiredness, feeling cold, weight gain, and other early symptoms of thyroid hemiagenesis when low hormone levels are the cause. Dosing may be increased or lowered gradually to keep blood test results in a healthy range.
Liothyronine is a faster-acting thyroid hormone that is only used in select situations, not as routine treatment for most people with thyroid hemiagenesis. Doctors may consider it when a very specific hormone balance issue is present, but levothyroxine is usually preferred because it gives steadier control.
Methimazole may be used if the remaining thyroid lobe becomes overactive and makes too much hormone. This drug lowers thyroid hormone production and can ease symptoms such as shakiness, sweating, and a racing heartbeat.
Propylthiouracil is another drug for an overactive thyroid, though it is used less often than methimazole. It may be chosen in certain situations, such as early pregnancy or when methimazole is not suitable.
Genetic influences
Many people first learn they have thyroid hemiagenesis after an ultrasound or scan for another thyroid concern, and a common next question is whether it could affect other family members too. Most cases appear to happen sporadically, with no clear inherited pattern, but research suggests that genes involved in early thyroid development may play a role in some people with thyroid hemiagenesis. Family history is one of the strongest clues to a genetic influence. Rare changes in genes such as PAX8, NKX2-1, or FOXE1 have been linked to thyroid development problems, although no single gene explains most cases. So if you are wondering whether thyroid hemiagenesis can run in families, the answer is sometimes, but often no definite genetic cause is found. If thyroid hemiagenesis shows up in more than one relative or occurs along with other birth differences, genetic counseling may help clarify whether an inherited factor is likely.
Is Thyroid hemiagenesis genetic?
Genetics may play a role in Thyroid hemiagenesis, but in many people no clear genetic cause is found. Rare familial cases suggest inherited factors can matter, likely alongside early developmental events before birth.
Having a genetic risk is not necessarily a diagnosis
Changes in genes are a normal part of human biology and are common throughout the general population. Most of these changes do not directly affect health and simply reflect natural human diversity. For many genes, having a change connected to a condition suggests a possible link rather than a certain diagnosis, especially when several genes and environmental factors are involved. In some cases, though, a single well-studied change in a specific gene can be enough to confirm a diagnosis.
Pharmacogenetics—how genetics influence drug effects
For many, treatment for thyroid hemiagenesis depends more on how well the remaining thyroid lobe works than on a specific gene result. If that lobe makes enough hormone, people with thyroid hemiagenesis may not need medicine at all, but if hormone levels are low, doctors usually prescribe levothyroxine and fine-tune the dose with blood tests and symptoms. Rare inherited changes in genes involved in thyroid development can sometimes run in families or appear alongside other thyroid problems, so family history may shape how closely someone is monitored. Not every difference in response is genetic, but age, body size, pregnancy, other medicines, and how well the body absorbs tablets often matter more when choosing the right dose. In other words, genetics may help explain why thyroid hemiagenesis happened, but day-to-day medication decisions are usually based on thyroid hormone levels rather than a pharmacogenetic test.
Interactions with other conditions
Many people with thyroid hemiagenesis go through daily life without obvious problems, but the single thyroid lobe can still develop the same illnesses seen in a two-lobed thyroid. Doctors call it a “comorbidity” when two conditions occur together. In practice, that means thyroid hemiagenesis may be found alongside an underactive thyroid, an overactive thyroid, thyroid inflammation such as Hashimoto disease, or thyroid nodules and enlargement. Some may find the first clue is not the missing lobe itself but early symptoms of thyroid disease, such as tiredness, feeling unusually cold or shaky, weight changes, or a new lump in the neck. Less often, people with thyroid hemiagenesis can also have thyroid cancer or nearby gland differences, especially involving the parathyroid glands that help control calcium. Because of this overlap, follow-up usually focuses on checking how well the remaining thyroid tissue is working and whether any other thyroid condition has developed.
How genes can cause diseases
Humans have more than 20,000 genes, each carrying out one or a few specific functions in the body. One gene instructs the body to digest lactose from milk, another tells the body how to build strong bones and another prevents the body's cells from beginning to multiply uncontrollably and developing into cancer. As all of these genes combined are the building instructions for our body, a defect in one of these genes can have severe health consequences.
Through decades of genetic research, we know the genetic code of any healthy/functional human gene. We have also identified that in certain positions on a gene, some individuals may have a different genetic letter from the one you have. We call these hotspots "Genetic Variations" or "Variants" in short. In many cases, studies have been able to show that having the genetic letter "G" in the position makes you healthy, but having the letter "A" in the same position disrupts the gene function and causes a disease. Genopedia allows you to view these variants in genes and summarizes all that we know from scientific research, which genetic letters (Genotype) have good or bad consequences on your health or on your traits.
What other conditions can cause similar symptoms to Thyroid hemiagenesis?
Many of the symptoms linked with thyroid hemiagenesis are the same as those caused by more common thyroid problems, especially an underactive thyroid, an overactive thyroid, inflammation of the thyroid, or thyroid nodules and swelling in the gland. In practice, doctors often first think about conditions such as Hashimoto's disease, Graves' disease, thyroiditis, or a goiter because these are much more common and can cause tiredness, weight changes, neck fullness, or changes in heart rate. Since many people with thyroid hemiagenesis have no symptoms at all, it is often found during tests being done for one of these other thyroid conditions.
History
Many people with thyroid hemiagenesis never know they have it until a scan is done for something else, such as a neck lump, thyroid blood test changes, or ongoing tiredness that prompts a closer look. In thyroid hemiagenesis, one half of the thyroid gland does not develop before birth, while the other half is present and may work normally.
For a long time, this was not something doctors could easily recognize during life. Before modern imaging, the thyroid could only be judged by touch, symptoms, and, in some cases, surgery or autopsy findings, so missing half of the gland was often overlooked. Not every early description was complete, yet together they helped build the picture doctors use today. As ultrasound, CT, and nuclear medicine scans became more widely available in the 20th century, thyroid hemiagenesis was identified more reliably, including in people who had no symptoms at all.
That shift mattered because it showed thyroid hemiagenesis is often a developmental difference rather than an illness by itself. Over time, doctors also learned that while many living with thyroid hemiagenesis have normal thyroid function, others may develop an underactive or overactive thyroid, nodules, or other thyroid conditions in the remaining lobe. This is one reason the early symptoms of thyroid hemiagenesis can be hard to pin down: often there are none, and when symptoms do happen, they usually come from how well the remaining thyroid tissue is working.
In recent decades, awareness has grown as imaging is used more often in both children and adults. Researchers later identified that thyroid hemiagenesis can have a genetic basis in some families, but its history in medicine mainly reflects better detection rather than a sudden change in the condition itself. Knowing this history helps explain why thyroid hemiagenesis may seem newly discovered to some families, even though it has likely always been present and simply went unrecognized for much of medical history.