For most adults, a normal TSH level falls between 0.27 and 4.2 mIU/L, though the exact range can shift depending on your age, whether you’re pregnant, and which lab processes your blood. TSH (thyroid-stimulating hormone) is the primary marker doctors use to evaluate thyroid function, but it’s not the only number that matters. Free T4 and Free T3 levels, and sometimes antibody tests, round out the full picture.
What TSH Actually Tells You
TSH is a hormone released by your pituitary gland to signal your thyroid to produce more hormones. When your thyroid is underactive, your pituitary pumps out extra TSH to compensate, pushing the number higher. When your thyroid is overactive, your pituitary dials TSH back, dropping the number lower. This inverse relationship is why a high TSH points to hypothyroidism and a low TSH points to hyperthyroidism.
Because TSH responds to even small changes in thyroid hormone output, it’s often the first test ordered and the most sensitive early indicator. A TSH result within range generally means your thyroid is producing enough hormone to meet your body’s needs.
Normal TSH Ranges by Age
Thyroid function changes across your lifespan, so “normal” isn’t one fixed number. Newborns naturally run much higher TSH levels that gradually decline through childhood. Here’s what labs typically consider normal:
- Newborns (up to 5 days): 0.7 to 15.2 mIU/L
- Infants 6 to 90 days: 0.72 to 11.0 mIU/L
- Babies 4 to 12 months: 0.73 to 8.35 mIU/L
- Children 1 to 6 years: 0.7 to 5.97 mIU/L
- Ages 12 to 20: 0.51 to 4.3 mIU/L
- Adults 21 and older: 0.27 to 4.2 mIU/L
Older adults tend to have slightly higher TSH levels naturally, which is why some endocrinologists accept a TSH up to 6 or 7 in patients over 70 without recommending treatment.
TSH Ranges During Pregnancy
Pregnancy shifts thyroid function significantly, especially in the first trimester when a hormone called hCG stimulates the thyroid directly. This naturally lowers TSH in early pregnancy. The trimester-specific ranges are:
- First trimester: 0.18 to 2.99 mIU/L
- Second trimester: 0.11 to 3.98 mIU/L
- Third trimester: 0.48 to 4.71 mIU/L
The Endocrine Society recommends keeping TSH below 2.5 mIU/L in the first trimester and below 3.0 mIU/L in the second and third trimesters. Uncontrolled thyroid dysfunction during pregnancy is linked to a higher risk of miscarriage and other complications, so pregnant women with borderline results are often monitored more closely than they would be otherwise.
Free T4 and Free T3
While TSH tells you how hard your pituitary is working, Free T4 and Free T3 measure the actual thyroid hormones circulating in your blood. T4 is the storage form your thyroid produces in large quantities, and your body converts it into T3, the active form that cells use for energy and metabolism. The “free” part means the test measures only the unbound, usable hormone rather than the total amount (some of which is attached to proteins and inactive).
Typical adult reference ranges are roughly 0.8 to 1.8 ng/dL for Free T4 and 2.3 to 4.2 pg/mL for Free T3, though these can vary between labs. Your doctor will usually order Free T4 alongside TSH if your TSH comes back abnormal, or if your symptoms don’t match your TSH result. Free T3 is checked less routinely but becomes important when hyperthyroidism is suspected, since some people overproduce T3 while their T4 stays relatively normal.
What “Optimal” Means vs. “Normal”
Lab reference ranges are built by measuring hormone levels in a large group of healthy people and defining the middle 95% as normal. That means 2.5% of perfectly healthy people will fall above the range and 2.5% below it, just by statistical design. This is one reason a single borderline result doesn’t automatically mean something is wrong.
There’s growing interest in where within the normal range your results fall. A large study published by the American Thyroid Association found that people whose TSH landed in the 60th to 80th percentile of the normal range had the lowest risk of death and heart disease. For Free T4, the sweet spot was the 20th to 40th percentile. In practical terms, this suggests that a TSH in the mid-to-upper portion of normal and a Free T4 in the lower-to-mid portion may be associated with the best long-term health outcomes. That said, these are population-level findings. Your ideal level depends on how you feel and your individual health profile, not just where a number lands on a bell curve.
Subclinical Thyroid Disease
Sometimes TSH drifts outside the normal range while Free T4 and Free T3 stay perfectly normal. This gray zone is called subclinical thyroid disease, and it’s more common than full-blown thyroid disorders.
Subclinical hypothyroidism means your TSH is above 4.5 mIU/L but your Free T4 is still normal. Most people in this category have a TSH between 4.5 and 10.0 mIU/L and may have no symptoms at all. Treatment decisions typically depend on how high TSH has risen, whether symptoms like fatigue or weight gain are present, and whether thyroid antibodies are elevated (suggesting the condition could progress). A TSH above 10 is more likely to be treated. Below 10, many doctors take a watch-and-wait approach with repeat testing every 6 to 12 months.
Subclinical hyperthyroidism is the opposite: TSH drops below 0.45 mIU/L while T4 and T3 remain normal. The range between 0.1 and 0.45 mIU/L is considered mildly low. This pattern can increase the risk of irregular heart rhythms and bone loss over time, particularly in older adults, so it’s not always as benign as it sounds.
Thyroid Antibody Tests
If your TSH is abnormal or borderline, your doctor may also check for thyroid antibodies. The most common is the thyroid peroxidase antibody (TPOAb), which targets an enzyme your thyroid needs to produce hormones. A normal TPOAb level is below 5.6 IU/mL. Levels above that suggest your immune system is attacking your thyroid, which is the hallmark of Hashimoto’s thyroiditis (the most common cause of hypothyroidism) or, less often, Graves’ disease.
Elevated antibodies with a normal TSH can indicate early autoimmune thyroid disease that hasn’t yet affected your hormone levels. About 2 to 5% of people with positive antibodies and normal TSH progress to overt hypothyroidism each year, so periodic monitoring is typically recommended.
What Can Throw Off Your Results
Biotin is the most common supplement that interferes with thyroid blood tests. It’s found in many hair, skin, and nail supplements, sometimes at doses of 5,000 to 10,000 mcg. Biotin can cause falsely high T4 and T3 readings and falsely low TSH, mimicking hyperthyroidism on paper even when your thyroid is functioning normally. The American Thyroid Association recommends stopping biotin at least two days before any thyroid blood draw.
Other factors that can shift your results include the time of day (TSH is highest in the early morning and lowest in the afternoon), recent illness or major stress, certain medications like steroids or anti-seizure drugs, and even fasting status. If you get a surprising result, your doctor will almost always repeat the test before making treatment decisions. A single out-of-range TSH doesn’t necessarily mean you have a thyroid problem, especially if you feel fine and the result is only slightly outside normal limits.