The hypothalamus produces Thyrotropin-releasing hormone (TRH) which stimulates the pituitary gland to release TSH. TSH stimulates the thyroid gland to secrete the hormones thyroxine (T4) and triiodothyronine (T3). The production of TSH is inhibited by the production of somatostatin by the hypothalamus.
The thyroid-stimulating hormone test measures the amount of TSH in the blood, to see if the thyroid gland is overactive or underactive. Higher than normal levels of TSH may indicate congenital hypothyroidism (cretinism), hypothyroidism or thyroid hormone resistance. Lower than normal levels of TSH may indicate hyperthyroidism.