testsTSH

Tests TSH refer to the measurement of thyroid-stimulating hormone in the blood. TSH is produced by the pituitary gland and regulates thyroid hormone production by the thyroid gland. The TSH test is a standard part of evaluating thyroid function and is often ordered together with free T4 to help classify thyroid conditions.

Procedure and reference ranges: A blood sample is drawn from a vein and analyzed by a laboratory immunoassay. Reference ranges vary by lab, but a common adult interval is about 0.4–4.0 mIU/L or 0.5–5.0 mIU/L. Values outside the reference range can indicate thyroid dysfunction, but interpretation should consider clinical context and may be aided by measuring free T4.

Interpretation: A high TSH typically signals hypothyroidism (insufficient thyroid hormone production or pituitary response), while a low TSH can indicate hyperthyroidism or pituitary suppression. TSH levels can be influenced by pregnancy, certain medications (for example, biotin, amiodarone, dopamine agonists, steroids), acute illness, and age. Central hypothyroidism may present with inappropriately normal or low TSH despite low thyroid hormones. Because of these factors, TSH is often interpreted alongside free T4 and sometimes free T3.

Uses and limitations: The TSH test is the first-line tool for screening and monitoring thyroid disorders, including autoimmune thyroid disease. It is most informative when combined with free T4. The test does not always distinguish all causes of thyroid dysfunction, and assay limitations or non-thyroidal illness can affect results.

Pre-test considerations: Fasting is not required, and timing is generally flexible, though TSH can show mild diurnal variation. Patients should inform clinicians about medications and supplements, especially biotin, estrogen, amiodarone, lithium, or other agents that can affect results.