A normal TSH level for most adults falls between 0.45 and 4.12 mIU/L. This range comes from a large U.S. population study (NHANES III) that measured TSH in people with no thyroid disease, no thyroid antibodies, and no family history of thyroid problems. Both the American Association of Clinical Endocrinologists and the American Thyroid Association endorse these limits as the standard reference range.
That said, “normal” isn’t one-size-fits-all. Your age, whether you’re pregnant, the time of day your blood was drawn, and even your ethnicity can shift what’s expected. Understanding these nuances helps you make sense of your results.
What TSH Actually Tells You
TSH (thyroid-stimulating hormone) is produced by your pituitary gland, a pea-sized structure at the base of your brain. Its job is to tell your thyroid how much hormone to make. When your thyroid is underactive, your pituitary pumps out more TSH to compensate, pushing the number higher. When your thyroid is overactive, TSH drops because the pituitary backs off.
This is why TSH works as a screening tool: it reflects how hard your body is working to keep thyroid hormones in balance. A high TSH suggests your thyroid isn’t keeping up. A low TSH suggests it’s doing too much.
The Standard Adult Range
Most labs list a normal range somewhere around 0.4 to 4.5 mIU/L, though the exact cutoffs vary slightly between testing methods. The widely cited NHANES III data, which excluded people with any thyroid risk factors, found that 95% of healthy adults had a TSH between 0.45 and 4.12 mIU/L.
Within that range, most healthy people cluster toward the lower end. A TSH of 1.0 to 2.5 mIU/L is where the majority of people without thyroid disease land, which is why some clinicians refer to this narrower window as the “optimal” range. This doesn’t mean a TSH of 3.5 is abnormal. It means that if you’re experiencing symptoms and your TSH is creeping toward the upper end, your doctor may want to investigate further rather than dismiss the result outright.
How Age Changes the Range
TSH naturally rises as you get older. In people over 80, the upper limit of normal at the 95th percentile exceeds 6.0 mIU/L, and in those over 90, it can reach 8.0 mIU/L. About 14% of people aged 85 and older have a TSH above 4.5 mIU/L, which would be flagged as high in a younger person but may be completely normal for their age.
This matters because overtreating a mildly elevated TSH in an older adult can cause real harm, including bone loss and heart rhythm problems. A TSH of 5.5 in a 30-year-old warrants a different conversation than the same number in an 80-year-old.
Children and Newborns
Babies are born with much higher TSH levels. In the first five days of life, a normal range is 0.7 to 15.2 mIU/L. This drops to 0.7 to 11.0 mIU/L by the first two months, then gradually narrows through childhood. By ages 6 to 10, the range is 0.6 to 4.8 mIU/L, and by adolescence (11 to 19), it settles to 0.5 to 4.3 mIU/L, close to adult levels.
TSH Ranges During Pregnancy
Pregnancy lowers TSH, especially in the first trimester, because a hormone produced by the placenta (hCG) stimulates the thyroid directly. The Endocrine Society recommends a TSH between 0.2 and 2.5 mIU/L in the first trimester and 0.3 to 3.0 mIU/L in the second and third trimesters.
These tighter ranges exist because even mildly elevated TSH during pregnancy is associated with an increased risk of miscarriage and developmental effects. If you’re pregnant or planning to become pregnant and already take thyroid medication, your dose will likely need adjustment early on.
Ethnicity and Individual Variation
TSH reference ranges also differ by race and ethnicity. Population data shows that TSH concentrations tend to be higher in White individuals compared to Black or Hispanic individuals. This means a TSH that falls in the middle of the standard range for one group might sit near the upper limit for another. Most labs use a single reference range regardless of ethnicity, so this is worth keeping in mind if your results are borderline.
What Happens Above and Below Normal
A TSH between roughly 4.5 and 10 mIU/L with normal thyroid hormone levels (T4) is classified as subclinical hypothyroidism. “Subclinical” means your thyroid is starting to struggle, but it’s still producing enough hormone to keep your blood levels in range. Many people with subclinical hypothyroidism have no symptoms at all. Others notice fatigue, weight gain, or feeling cold. Whether treatment is needed depends on how high the TSH is, whether you have symptoms, and whether thyroid antibodies are present.
When TSH rises above 10 mIU/L, treatment is more consistently recommended because the risk of progressing to full-blown hypothyroidism increases significantly. At that level, T4 is often low as well, and symptoms are more likely.
On the other end, a TSH below 0.4 mIU/L suggests hyperthyroidism, where the thyroid is overproducing hormones. Symptoms can include a rapid heartbeat, unintentional weight loss, anxiety, and tremors. A suppressed TSH near zero is more concerning than one that’s just slightly below range.
Why Your TSH Can Fluctuate
TSH follows a circadian rhythm. Levels peak between 2:00 and 4:00 a.m. and hit their lowest point during the afternoon. This means a blood draw at 8:00 a.m. will typically produce a higher TSH than one taken at 2:00 p.m. The difference can be large enough to push a borderline result above or below the reference range, potentially changing whether you’re diagnosed or how your medication dose is adjusted.
For this reason, consistent timing matters. If you’re monitoring TSH over time, try to get your blood drawn at roughly the same time of day each visit, ideally in the morning. Seasonal variation also plays a role, with TSH tending to run slightly higher in winter months.
Biotin Can Skew Your Results
Biotin, a B vitamin found in many hair, skin, and nail supplements, can interfere with common TSH assays and produce misleading results. The American Thyroid Association recommends stopping biotin supplements at least two days before any thyroid blood test. Interference has been reported at blood concentrations as low as 10 ng/mL, a level easily reached with high-dose supplements (5,000 to 10,000 mcg daily). Standard multivitamins with small amounts of biotin are generally not a concern, but it’s worth mentioning any supplements to whoever orders your test.
One Number Isn’t the Whole Picture
TSH is a screening tool, not a diagnosis on its own. If your TSH comes back abnormal, the next step is usually measuring free T4 (the active thyroid hormone) and sometimes T3 to see whether your thyroid is actually producing too much or too little. Thyroid antibody tests can help determine whether an autoimmune condition like Hashimoto’s thyroiditis is driving the problem.
Your symptoms matter too. Some people feel perfectly fine with a TSH of 4.0, while others feel sluggish at 3.0. A single TSH result that’s slightly outside the reference range, especially if you had your blood drawn in the afternoon or were recently ill, may not mean anything at all. A repeat test a few weeks later, drawn in the morning, often clarifies the picture.