TSH — Normal Range & Interpretation

Clinical Context

TSH (thyroid-stimulating hormone) is produced by the anterior pituitary and regulates thyroid hormone synthesis through negative feedback with T3 and T4. It serves as the most sensitive first-line screen for thyroid dysfunction because small shifts in circulating thyroid hormone produce logarithmic changes in TSH, making it the preferred test for screening, diagnosis, and monitoring thyroid replacement therapy.

Elevated TSH reflects primary hypothyroidism, with Hashimoto thyroiditis as the leading cause in the United States, followed by post-ablative or post-surgical states, iodine deficiency, and lithium or amiodarone use. Suppressed TSH indicates primary hyperthyroidism from Graves disease, toxic multinodular goiter, thyroiditis, or exogenous levothyroxine overreplacement. Central hypothyroidism from pituitary disease produces low or inappropriately normal TSH with low free T4, requiring pairing of TSH with free T4 for accurate interpretation.

High-yield for NP boards: the classic patterns — high TSH with low free T4 as overt hypothyroidism, high TSH with normal free T4 as subclinical hypothyroidism, and low TSH with high free T4 as overt hyperthyroidism. Expect questions on levothyroxine dose titration using TSH rechecked at six to eight weeks, pregnancy-specific targets under 2.5 in the first trimester, and screening intervals in older adults and patients with autoimmune disease.

Related Labs

• Free T4 — ng/dL
• Free T3 — pg/mL
• Total T4 — μg/dL

Sources

• MedlinePlus