A hormone released by the anterior lobe of the pituitary gland, TSH acts to stimulate the thyroid gland to release the thyroid hormones thyroxine (T4) and thiiodothyronine (T3).
The release of TSH is affected by negative feedback depending on the levels of circulating thyroid hormones. When the levels of thyroid hormones reach more than 1.75 above normal, the release of TSH is turned off.
If the levels of only one thyroid hormone were to be measured in a blood test to test thyroid function, the most useful one to measure would be the TSH level.
TSH secretion is also affected by TRH.