TSH, or thyroid-stimulating hormone, is a hormone produced by the pituitary gland that controls how much thyroid hormone your thyroid makes. For most adults, a normal TSH level falls between 0.27 and 4.2 uIU/mL. Your doctor uses this single blood test as the primary window into whether your thyroid is working properly, because TSH shifts before other thyroid hormones do, making it the earliest indicator of a problem.
How TSH Works
Your body regulates thyroid function through a feedback loop that starts in the brain. The hypothalamus releases a signaling hormone that tells the pituitary gland to produce TSH. TSH then travels through the bloodstream to the thyroid gland, where it triggers the production and release of two thyroid hormones: T3 and T4. These hormones control your metabolism, body temperature, heart rate, and energy levels.
When T3 and T4 levels rise high enough, they signal back to the brain to slow down TSH production. When thyroid hormone levels drop, TSH production ramps up. This is why TSH moves in the opposite direction of thyroid hormones: a high TSH usually means your thyroid isn’t making enough hormone, and a low TSH usually means it’s making too much.
Normal TSH Ranges by Age
The standard adult reference range is 0.27 to 4.2 uIU/mL, but “normal” shifts with age. An analysis of the NHANES III database found the upper limit of normal was 3.5 mIU/mL for adults in their 20s, rising to 4.5 mIU/mL for those aged 50 to 70, and 7.5 mIU/mL for people over 80. This means a TSH of 5.0 in an 85-year-old is perfectly normal, while the same result in a 25-year-old would be flagged as elevated.
Children and teens also have different ranges. Newborns in their first month can have TSH levels anywhere from about 0.7 to 18 mIU/L, which drops considerably over the first year. By ages 6 to 10, the typical range narrows to roughly 0.8 to 5.4 mIU/L. Through adolescence, TSH continues to decline, and by ages 15 to 18, the range settles closer to adult levels at about 0.5 to 4.3 mIU/L.
TSH During Pregnancy
Pregnancy lowers TSH naturally, especially in the first trimester when pregnancy hormones stimulate the thyroid directly. The American Thyroid Association has recommended trimester-specific ranges: 0.1 to 2.5 mIU/L in the first trimester, 0.2 to 3.0 in the second, and 0.3 to 3.5 in the third. Both the Endocrine Society and European Thyroid Association suggest keeping TSH below 2.5 mIU/L in the first trimester and below 3.0 in the second and third trimesters for women being treated for hypothyroidism.
These tighter ranges matter because thyroid hormones play a critical role in fetal brain development, particularly early in pregnancy before the baby’s own thyroid begins functioning.
“Optimal” vs. Normal TSH
There’s an important distinction between the lab reference range and what many clinicians consider the ideal target. The standard range extends up to about 4.2 or 4.5 mIU/mL, but the “most normal” range is likely between 0.5 and 2.5 mIU/mL. This narrower window is where most healthy people without thyroid disease cluster, and it’s the target range doctors aim for when adjusting medication in people already diagnosed with hypothyroidism.
That said, a TSH of 4.2 in someone with no symptoms doesn’t necessarily mean they need treatment. It might, however, prompt a doctor to increase the dose for someone already on thyroid medication. The diagnostic threshold and the treatment target are not the same thing.
What High TSH Means
An elevated TSH signals that your thyroid isn’t producing enough hormone, so your pituitary is working harder to compensate. This is the hallmark of hypothyroidism. When TSH is elevated but your T4 level is still in the normal range, the condition is called subclinical hypothyroidism. You may have no symptoms at all, or you may notice subtle changes that develop slowly over months or years.
When hypothyroidism becomes more pronounced, symptoms tend to include fatigue, weight gain, constipation, dry skin, sensitivity to cold, a puffy face, hoarse voice, muscle weakness and aches, and heavier or irregular periods. These symptoms develop gradually, which is why many people attribute them to aging or stress before getting tested. Some people with a TSH above 10 mIU/L still have a normal T4 level, which reflects how individual the relationship between these hormones can be.
What Low TSH Means
A suppressed TSH means the pituitary has dialed back its signal because there’s already too much thyroid hormone circulating. This points to hyperthyroidism, or an overactive thyroid. Common causes include autoimmune conditions like Graves’ disease, thyroid inflammation that causes stored hormone to leak into the bloodstream, and thyroid nodules that produce hormone independently.
Symptoms of hyperthyroidism tend to be the opposite of hypothyroidism: unintentional weight loss, rapid or irregular heartbeat, heart palpitations, anxiety, trembling hands, increased hunger, heat sensitivity, more frequent bowel movements, trouble sleeping, and thinning hair or skin. Older adults sometimes present differently, with depression, fatigue, and weight loss as their only noticeable symptoms, which can make the condition easy to miss without a blood test.
What Can Affect Your Test Results
If you take biotin supplements (also marketed as vitamin B7 or sold in hair, skin, and nail formulas), they can interfere with the lab assay used to measure TSH. Excess biotin in your blood competes with the test’s internal chemistry and produces a falsely low TSH reading, which could make it look like you have hyperthyroidism when you don’t. In documented cases, stopping biotin for one week was enough for TSH and T4 measurements to return to their true values. If you’re taking biotin at high doses, let your doctor know before a thyroid panel so they can advise you to pause it beforehand.
TSH also fluctuates throughout the day, peaking in the early morning hours and dipping in the afternoon. Most labs draw thyroid panels in the morning for consistency. Illness, stress, certain medications (particularly steroids and some seizure drugs), and even skipping a meal can temporarily shift your TSH, which is why a single abnormal result is often rechecked before any diagnosis is made.