A simple, regular blood test is used to guide treatment for hypothyroidism and monitor the condition. The main job of the thyroid gland is to make the hormone thyroxine, also known as T4 because it has four iodine molecules.
The thyroid also makes the hormone triiodothyronine, known as T3 because it has three iodine molecules, but in smaller amounts, explains Cathy Doria-Medina, MD , an endocrinologist with HealthCare Partners Medical Group in Torrance, California. The pituitary gland at the base of the brain controls hormone production in your body. It makes thyroid-stimulating hormone TSH , which tells the thyroid gland how much T4 and T3 to produce.
The TSH level in your blood reveals how much T4 your pituitary gland is asking your thyroid gland to make. Measure content performance. Develop and improve products. List of Partners vendors. This applies whether you're living with thyroid disease or having your first thyroid test. TSH test results can be confusing.
A high TSH level points to hypothyroidism underactive thyroid. A low TSH might mean hyperthyroidism overactive thyroid. But there are exceptions to these interpretations, as well as what a "normal" level means for you. You will also learn about testing for thyroid disorder and a number factors that may impact your results.
TSH is a hormone that acts as a messenger to the thyroid gland. When released by the pituitary gland—a small gland at the base of the brain—it stimulates the production of more thyroid hormone.
A high TSH suggests your thyroid is underactive hypothyroid and not doing its job of producing enough thyroid hormone. A low TSH suggests your thyroid is overactive hyperthyroid and producing excess thyroid hormone.
TSH levels seem counterintuitive. Why does a high TSH mean you have an underactive thyroid gland? And why do low levels mean it's overactive? To answer that, you have to know that TSH and thyroid hormones are different things. Thyroid hormones are:. Your body uses thyroid hormones to regulate all kinds of processes. That includes how fast you burn calories, your heart rate, and many other automatic functions.
When these hormones are low, your pituitary makes more TSH. That signals the thyroid to increase production. When levels are high, the pituitary releases less TSH. That signals the thyroid to slow down. This is why high TSH indicates low thyroid function and vice versa.
Understanding when and why thyroid hormone is released or not helps clear up what a low TSH and a high TSH level reveal about your thyroid's function.
This abbreviation stands for milliunits per litre, with one milliunit equating to one-thousandth of a unit. The normal range for TSH is generally between 0. As with most medical conditions and tests, however, there are exceptions to this rule.
Thyroid hormone levels that are considered normal may be abnormal for you under certain conditions. It's for this reason that healthcare providers have varied opinions about what the optimal TSH level should be. While most laboratories define a normal TSH as between roughly 0. The reasoning behind this is that the vast majority of adults without thyroid disease have a TSH value between 0.
In addition, some healthcare providers believe older patients should have a TSH level that is greater than 4. Healthcare providers can avoid some of this controversy simply by looking at each person as an individual. For example, a person who still has significant symptoms of hypothyroidism at a TSH of 4. In contrast, someone who has health risks—such as heart disease or osteoporosis—may benefit from having a goal TSH that is higher, perhaps around 5.
In pregnancy, TSH should not be allowed to rise above 2. That protects the health of both the baby and mother. A normal TSH value is generally defined as between.
But opinions about this vary, since everyone has a personal "normal" that depends on their age and overall health. For example, your target level may be higher if you have heart disease or lower if you are pregnant.
A high TSH means different things depending on whether a person has known thyroid disease or not. A high TSH in people who are not undergoing thyroid disease treatment usually indicates the presence of primary hypothyroidism. This is by far the most common form of hypothyroidism , and it occurs because the thyroid gland produces an inadequate amount of thyroid hormones. Preparing a list of questions will help you make the most of your time with your doctor.
For hypothyroidism, some basic questions to ask include:. Mayo Clinic does not endorse companies or products. Advertising revenue supports our not-for-profit mission. This content does not have an English version. This content does not have an Arabic version. Diagnosis In general, your doctor may test for an underactive thyroid if you are feeling increasingly tired, have dry skin, constipation and weight gain, or have had previous thyroid problems or a goiter.
Blood tests Diagnosis of hypothyroidism is based on your symptoms and the results of blood tests that measure the level of TSH and sometimes the level of the thyroid hormone thyroxine. More Information Hypothyroidism diet Hypothyroidism: Can calcium supplements interfere with treatment? Hypothyroidism: Should I take iodine supplements? Soy: Does it worsen hypothyroidism? Show more related information.
Request an Appointment at Mayo Clinic. More Information Coconut oil: Can it cure hypothyroidism? Share on: Facebook Twitter. Show references AskMayoExpert. Rochester, Minn. Chaker L, et al.
The Lancet. American Thyroid Association. In some people, thyroid conditions are linked with sexual dysfunction. This may affect more males than females. Blood TSH levels tend to increase as a person gets older.
According to the authors of one study , the percentage of people with TSH levels between 0. Pregnancy hormones naturally increase the levels of certain thyroid hormones in the blood. This is essential for the development of the fetal brain and nervous system.
At the same time, the levels of TSH in the blood decrease. As a result, doctors use lower reference ranges for pregnant women. The lower TSH range is decreased by around 0. Levels of TSH in the blood increase gradually during the second and third trimesters, but they remain lower than normal levels in women who are not pregnant.
Doctors carefully monitor TSH levels throughout pregnancy. Having unusually high or low levels can affect the risk of miscarriage and cause pregnancy-related complications , such as:.
The following table shows TSH levels for children by age, according to a study from based on data from healthy children:. High TSH levels indicate hypothyroidism. People develop hypothyroidism when their thyroid produces low levels of hormones. When the thyroid gland does not produce enough hormones, the pituitary gland produces more TSH to compensate.
Symptoms of hypothyroidism include:. Low TSH levels indicate hyperthyroidism. This is also known as an overactive thyroid. If the thyroid gland is secreting levels of hormones that are too high, the pituitary gland produces less TSH. Symptoms of hyperthyroidism include:. The TSH test involves a healthcare professional drawing blood from a vein in the inner arm.
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