Thyroid-stimulating hormone (TSH) also called thyrotropin, is a hormone that the pituitary gland releases to trigger the thyroid to produce and release its own hormones (T4 and T3). High TSH levels usually indicate hypothyroidism, and low TSH levels usually indicate hyperthyroidism.

The test is ordered for differential diagnosis of primary, secondary, and tertiary hypothyroidism. Also useful in screening for hyperthyroidism. This assay allows adjustment of exogenous thyroxine dosage in hypothyroid patients and in patients on suppressive thyroxine therapy for thyroid neoplasia.