For most adults, TSH levels should fall between 0.27 and 4.2 mIU/L. That’s the standard reference range used by most laboratories, though your ideal number within that window depends on your age, whether you’re pregnant, and other health factors. TSH, or thyroid-stimulating hormone, is the single most common blood test for checking thyroid function, and understanding where your result lands can help you have a more productive conversation with your doctor.
The Standard Adult Range
Laboratories typically flag TSH results as abnormal if they fall outside the 0.27 to 4.2 mIU/L range. A TSH above that upper limit suggests your thyroid is underactive (hypothyroidism), because your brain is pumping out extra TSH trying to coax the thyroid into producing more hormone. A TSH below the lower limit suggests the opposite: an overactive thyroid (hyperthyroidism), where the brain dials back TSH because there’s already too much thyroid hormone circulating.
Large population studies have helped refine these numbers. Data from the National Health and Nutrition Examination Survey, which screened a disease-free population, placed the upper limit of normal at about 4.5 mIU/L. When researchers narrowed the group further to exclude anyone with thyroid antibodies, pregnancy, or medications known to affect the thyroid, that upper limit dropped slightly to 4.12 mIU/L.
Is There an “Optimal” Range?
You may have seen claims that the truly healthy TSH sits between 0.5 and 2.5 mIU/L, well below the upper boundary of the standard range. The evidence here is more nuanced than it first appears. A large meta-analysis that pooled individual patient data found that people whose TSH fell in the 60th to 80th percentile of the normal range had the lowest risk of death and heart disease. That’s actually the upper-middle portion of the range, not the lower end.
People in the lowest 20th percentile of normal TSH had a 9% higher risk of death from any cause and a 7% higher risk of cardiovascular death compared to the 60th-to-80th percentile group. In other words, a very low-normal TSH wasn’t protective. The takeaway: a number that falls comfortably within the reference range is generally a good sign, and chasing an artificially low target isn’t supported by the best available data.
How Age Shifts the Range
TSH naturally rises as you get older. Children tend to run higher than adults, and after midlife the upper boundary creeps up further, especially after age 50 in women and 60 in men. To put a number on it: the upper limit of normal for a 50-year-old woman is about 4.0 mIU/L, but by age 90 that limit increases by roughly 50%, reaching around 6.0 mIU/L.
This matters because a TSH of 5.5 in a 75-year-old may be perfectly normal physiology, while the same result in a 30-year-old would warrant a closer look. Some older adults end up on thyroid medication they don’t need because their doctor used the same reference range applied to younger patients. If you’re over 60 and your TSH is mildly elevated, it’s worth asking whether age-adjusted norms were considered.
TSH Targets During Pregnancy
Pregnancy changes the game. The Endocrine Society recommends keeping TSH between 0.2 and 2.5 mIU/L during the first trimester, then between 0.3 and 3.0 mIU/L for the second and third trimesters. These tighter targets exist because even mildly low thyroid function in early pregnancy is associated with a higher risk of miscarriage and developmental concerns for the baby.
For women who are planning to become pregnant or who are already pregnant, treatment with thyroid hormone replacement is generally advised if TSH rises above 2.5 mIU/L, particularly if thyroid antibodies are present. This is one of the clearest situations where the standard lab range doesn’t apply and a narrower target is clinically important.
When a Mildly High TSH Needs Treatment
A TSH that’s above the reference range but below 10 mIU/L, with normal levels of actual thyroid hormone, is called subclinical hypothyroidism. It’s common, and whether to treat it is one of the more debated questions in endocrinology.
The strongest evidence favors starting thyroid hormone replacement when TSH exceeds 10 mIU/L. Below that threshold, the picture gets murkier. Many clinicians consider treatment when TSH reaches 7 or 8 mIU/L, especially in younger patients, those with symptoms like fatigue and weight gain, or those with high cholesterol that could partly stem from sluggish thyroid function. For TSH between 5 and 10 mIU/L without clear symptoms, watchful waiting with repeat testing in a few months is a reasonable approach.
TSH After Thyroid Cancer
People who’ve had their thyroid removed for cancer have historically been given enough thyroid hormone replacement to push their TSH well below normal, sometimes to undetectable levels. The idea was to starve any remaining cancer cells of the growth signal that TSH provides.
Updated 2025 guidelines from the American Thyroid Association have pulled back on this approach. Complete TSH suppression to undetectable levels is no longer recommended for any patient group. TSH-lowering therapy is now reserved mainly for people with evidence of possible persistent cancer, such as detectable tumor markers or suspicious imaging findings. For everyone else, keeping TSH within or just below the normal range is considered sufficient. This shift reflects growing recognition that years of suppressed TSH carry their own risks, including bone loss and heart rhythm problems.
What Can Throw Off Your Results
TSH follows a daily rhythm. Levels peak in the early morning hours and drop later in the day, with the total swing over 24 hours roughly doubling the lowest reading. Blood draws are typically done between 7:00 and 10:00 a.m. for this reason. If you had blood drawn at 3:00 p.m. one visit and 8:00 a.m. the next, the difference in results could reflect timing rather than a real change in thyroid function.
Biotin supplements, commonly found in hair, skin, and nail formulas, can interfere with the lab assay itself and produce misleading results. The American Thyroid Association recommends stopping biotin for at least two days before any thyroid blood test.
Several medications also affect TSH independently of actual thyroid health. Steroids like prednisone can suppress TSH, making a hypothyroid patient’s results look normal when they aren’t. Lithium, often used for bipolar disorder, tends to push TSH upward. Amiodarone, a heart rhythm medication, can swing TSH in either direction. If you take any of these, your doctor should interpret your TSH with that context in mind.
Making Sense of Your Number
A single TSH value is a snapshot, not a diagnosis. Because levels fluctuate with time of day, stress, illness, and medication, an abnormal result is typically confirmed with a second test before any treatment decisions are made. When your TSH comes back outside the normal range, your doctor will usually order free T4 (and sometimes free T3) to see whether the thyroid hormones themselves are actually low or high, or whether the TSH shift is an isolated finding.
If your TSH is within range and you still feel lousy, it’s reasonable to ask about testing free T4 and thyroid antibodies, which can reveal early autoimmune thyroid disease before TSH moves. But for most people, a TSH that sits comfortably between about 1.0 and 4.0 mIU/L, adjusted for age, indicates a thyroid that’s doing its job.