TSH test: what it measures, normal levels by age and pregnancy, and what high or low means

TSH is made by the pituitary gland at the base of the brain. Its job is to tell the thyroid to release thyroxine (T4) and triiodothyronine (T3), the hormones that set the pace of metabolism in nearly every cell. The system runs on feedback: when thyroid hormone is low, the pituitary raises TSH to push the gland harder; when thyroid hormone is high, TSH falls.

That feedback is what makes TSH so useful. The relationship is logarithmic — a halving of free T4 roughly doubles or triples TSH — so TSH usually drifts out of range before T4 does. It is why TSH is the first-line thyroid test in essentially every guideline and why a normal TSH, in a person with a healthy pituitary, is strong evidence the thyroid is working normally.

The test is reported in milli-international units per liter (mIU/L; some labs write µIU/mL, which is the same number). It is a simple blood draw and is frequently bundled with free T4, and sometimes free T3 and thyroid antibodies, in a thyroid panel.

TSH moves opposite to thyroid hormone: a high TSH usually means the thyroid is underactive (hypothyroidism) and a low TSH that it is overactive (hyperthyroidism). High-TSH symptoms develop over months:

Low-TSH symptoms run the other way: unintended weight loss, a rapid or irregular heartbeat and palpitations, heat intolerance and sweating, anxiety, irritability and tremor, trouble sleeping, frequent bowel movements, muscle weakness, lighter or missed periods, and in Graves' disease eye irritation or bulging.

A mildly high TSH with a normal free T4 — subclinical hypothyroidism — frequently causes no symptoms and is usually repeated in six to twelve weeks before treatment is considered; a substantial share of such results return to normal on their own. Recovery from an illness, biotin supplements and recent dose changes can all move TSH.

TSH is tested when symptoms suggest the thyroid is under- or overactive. Underactive (hypothyroid) symptoms include fatigue, feeling cold, weight gain without a change in habits, constipation, dry skin, hair thinning, a slowed heart rate, heavier periods and low mood. Overactive (hyperthyroid) symptoms include unexplained weight loss, heat intolerance, palpitations, tremor, anxiety, insomnia, frequent bowel movements and lighter periods.

It is also checked in people at higher risk even without clear symptoms: anyone with a goiter or thyroid nodules, a family history of thyroid disease, another autoimmune condition such as type 1 diabetes or celiac disease, previous neck radiation or thyroid surgery, Down syndrome or Turner syndrome, and people taking medications known to affect the thyroid — amiodarone, lithium, interferon and cancer immunotherapy in particular.

Women planning a pregnancy, pregnant, or having trouble conceiving are commonly tested because even mild thyroid dysfunction can affect fertility and pregnancy, and pregnancy has its own reference ranges. Anyone taking levothyroxine needs TSH to guide the dose. Guidelines disagree on whether adults who feel well should be screened routinely; the practical answer is that a TSH is inexpensive and a reasonable check when symptoms or risk factors are present.

No fasting is required for TSH, and you can have it drawn at any time of day. TSH does have a mild daily rhythm — highest late at night and in the early morning, lowest in the afternoon — so if you are tracking a borderline value over time, drawing at a similar hour each time makes the comparison cleaner.

Two preparation details matter more than timing. First, biotin: high-dose biotin supplements (common in hair, skin and nail products) can interfere with the immunoassays many labs use for TSH and free T4, producing falsely low TSH and falsely high T4. Stop biotin for at least two days before the draw, longer for very large doses. Second, if you take levothyroxine, take it after the blood draw on test day or note the timing for whoever reads the result; the timing affects free T4 more than TSH, but consistency helps.

You can order a TSH test online through TestWell, have blood drawn at a nearby Quest or Labcorp patient service center after a physician reviews the order, and receive results in your portal, typically within a few business days. If the result is abnormal, a free T4 — and often TPO antibodies — is the usual next step, and many people order the thyroid panel from the start to avoid a second visit.

Most US laboratories report an adult reference range of roughly 0.45 to 4.5 mIU/L, and that is the range TestWell's catalog uses. The exact limits vary slightly by lab and method, so your report's printed range takes precedence. There has been a long debate about whether the upper limit should be lower (around 2.5 to 3.0 mIU/L) because some people with values in the upper part of the range have early autoimmune thyroid disease; most guidelines have kept the wider range, because many people with a TSH of 3 to 4.5 never develop hypothyroidism.

Age changes what counts as normal. Population studies show the TSH distribution shifts upward in older adults, and a TSH of 5 to 7 mIU/L in a person over 70 with a normal free T4 is often within that age group's normal spread rather than a sign of disease. This is one of the reasons mild elevations in older adults are usually rechecked rather than treated.

Pregnancy lowers TSH, particularly in the first trimester, because the pregnancy hormone hCG stimulates the thyroid directly. The American Thyroid Association advises trimester- and population-specific ranges where a lab has them, and where it does not, an upper limit of roughly 4.0 mIU/L in the first trimester with a lower limit around 0.1 mIU/L. Women being treated for hypothyroidism are usually checked more frequently during pregnancy.

TSH tells you the pituitary's opinion; free T4 tells you the actual thyroid hormone level. Read together, they sort most results into a handful of patterns. A high TSH with a low free T4 is overt hypothyroidism. A high TSH with a normal free T4 is subclinical hypothyroidism, which is frequently rechecked in 6 to 12 weeks before any decision, because a meaningful share of mild elevations normalize on their own. A low TSH with a high free T4 or free T3 is overt hyperthyroidism; a low TSH with normal hormone levels is subclinical hyperthyroidism.

The exceptions are the patterns where TSH and T4 move in the same direction. A low or inappropriately normal TSH with a low free T4 points to a pituitary or hypothalamic problem (central hypothyroidism) rather than the thyroid itself. A high TSH with a high free T4 is rare and raises the question of assay interference, a TSH-secreting pituitary tumor or thyroid hormone resistance — all reasons the result is repeated, often by a different method, before anything else happens.

Recovering from a significant illness can push TSH up for several weeks as the body resets, and being acutely ill can push it down. Medications matter: glucocorticoids, dopamine and high-dose aspirin lower TSH; amiodarone and lithium raise it over time; metformin nudges it down slightly in people on levothyroxine. Biotin interference, discussed above, can make TSH read falsely low. Assay interference from certain antibodies is uncommon but real, which is why a result that makes no clinical sense is rerun on a different platform.

Poor levothyroxine absorption is a common, fixable cause of a high TSH in someone already on treatment: taking the tablet with food, coffee, calcium, iron or a proton-pump inhibitor can reduce how much is absorbed. Finally, TSH varies from day to day by about 10–20% in the same person, so small shifts within the range are noise, not a trend.

The next step is almost always a free T4 (and free T3 if TSH is low) to place the result in one of the patterns above, plus TPO antibodies when TSH is high, because Hashimoto's thyroiditis is the most common cause of an underactive thyroid and antibodies predict who progresses. A mildly abnormal TSH with normal hormone levels is typically repeated in 6 to 12 weeks rather than acted on immediately.

Clearly abnormal results, symptoms such as a racing heart, chest pain or confusion, a visibly enlarged thyroid, or any abnormal result during pregnancy warrant prompt clinician review rather than a wait-and-repeat approach. The interpretation sections below walk through the common causes, symptoms and follow-up tests for a high and a low TSH.