TSH is a hormone synthesized in the pituitary gland that stimulates the production of T4 (thyroxine) and T3 (triiodothyronine) hormones by the thyroid gland. Abnormal blood levels of TSH should be interpreted in light of T4 and T3 levels.
High blood levels of TSH indicate that the thyroid gland is not active enough or is not responding well to TSH stimulation. In individuals being treated for hypothyroidism, a high TSH level indicates that the treatment is not adequate and that hormone replacement doses should be increased. In individuals being treated for hyperthyroidism, the development of high levels of TSH indicates that the dose of antithyroid medication is too high and needs to be reduced. Very rarely, a high TSH level can be associated with pituitary gland problems such as a tumour producing TSH.
A low TSH level may result from an overactive thyroid gland (hyperthyroidism), too high of a dose of thyroid hormones administered to treat hypothyroidism, or undertreated hyperthyroidism. Very rarely, pituitary gland damage can prevent the gland from producing sufficient quantities of TSH.