Normal Reference Range for Hypothyroidism Indicators

For patients with hypothyroidism, the “normal” reference ranges for thyroid function tests depend primarily on whether the patient is pregnant. During pregnancy, the target TSH level differs by trimester: in the first trimester, TSH should be < 2.5 mIU/L; in the second and third trimesters, TSH should be < 3.0 mIU/L. So, what are the normal reference ranges for thyroid function tests in hypothyroidism?

Normal Reference Ranges for Thyroid Function Tests in Hypothyroidism

In general, all thyroid function parameters should be maintained within the standard adult reference ranges. Specifically, free T3 (FT3) should be maintained between 3.1–6.8 pmol/L, and free T4 (FT4) between 12–22 pmol/L. Because thyroid hormone demand increases significantly during pregnancy, treatment for overt hypothyroidism in non-pregnant adults is typically initiated when TSH exceeds 8 mIU/L.

Hypothyroidism can be classified into primary, pituitary (central), hypothalamic (central), and peripheral types—each diagnosed based on patterns of TSH and thyroid hormone levels. Primary hypothyroidism is diagnosed when TSH is markedly elevated and both FT4 and FT3 are below the normal reference range. Because thyroid hormone demand increases substantially during pregnancy, treatment for overt hypothyroidism in non-pregnant adults is generally initiated when TSH exceeds 8 mIU/L.

For non-pregnant hypothyroid patients, thyroid function test results are considered acceptable if they fall within the local laboratory’s established reference range. Note that different assay kits may yield slightly varying reference values; however, when units are standardized to mIU/L for TSH, interpretation should strictly follow the laboratory’s published normal range. We hope this information is helpful!