What is a normal TSH level, and why do labs disagree?

A normal TSH in most United States laboratories falls somewhere between about 0.4 and 4.5 mIU/L. The exact numbers on your report come from the assay your laboratory runs and the reference population that assay was validated against, which is why two labs can call the same blood sample normal and borderline. The American Thyroid Association 2014 hypothyroidism guideline treats a TSH within the age-appropriate laboratory reference range as the treatment target.

Why TSH reference ranges differ between laboratories

TSH reference ranges are built by measuring a large group of people judged to be free of thyroid disease and taking the middle 95 percent of results. Different manufacturers use different capture antibodies and different calibrators, so the numbers do not transfer between platforms. Reference populations also differ by iodine intake, ethnicity and how carefully people with thyroid antibodies were excluded. In the NHANES III disease-free population, the geometric mean TSH was 1.40 mIU/L, well below the middle of most reported ranges, because the distribution of TSH is skewed rather than symmetric.

The 2.5 mIU/L upper limit is contested

Some clinicians and many wellness sites state that TSH above 2.5 mIU/L is abnormal. This proposal came from laboratory medicine groups who argued that people with unrecognised early autoimmune thyroid disease inflate the upper end of the reference range. It has not been adopted by the American Thyroid Association, the Endocrine Society or the American Association of Clinical Endocrinology for non-pregnant adults, and applying it would reclassify a large share of healthy people as hypothyroid. The one place a 2.5 mIU/L figure does appear in guidelines is preconception care: the 2026 American Thyroid Association pregnancy guideline calls a TSH between 0.5 and 2.5 mU/L a reasonable target for a woman already taking levothyroxine who is planning pregnancy.

TSH rises with age

TSH drifts upward with age in people who never develop thyroid disease. Applying a young adult upper limit to an eighty-year-old converts normal ageing into a diagnosis. A 2025 Cleveland Clinic Journal of Medicine review describes targets of 4 to 6 mIU/L for patients in their seventies and a watchful approach for TSH at or below 10 mIU/L above age eighty.

Bring the actual printed reference range with any TSH result, not the number alone. If a clinician tells you your TSH of 3.2 mIU/L is abnormal, ask which guideline that threshold comes from and what the plan is if treatment does not change how you feel.