Prior to the availability of sensitive TSH assays, thyrotropin releasing hormone or TRH stimulation tests were relied upon for confirming and assessing the degree of suppression in suspected hyperthyroidism.
In primary hyperthyroidism TSH are low and TRH administration induces little or no change in TSH levels • In hypothyroidism due to end organ failure, administration of TRH produces a prompt increase in TSH • In hypothyroidism due to pituitary disease (secondary hypothyroidism) administration of TRH does not produce an increase in TSH • In hypothyroidism due to hypothalamic disease (tertiary hypothyroidism), administration of TRH produces a delayed (60–120 minutes, rather than 15–30 minutes) increase in TSH The TRH test involves administration of a small amount of TRH intravenously,[1] following which levels of TSH will be measured at several subsequent time points using samples of blood taken from a peripheral vein.
After 30 minutes blood is drawn again and the levels of TSH are measured and compared to the baseline.
[citation needed] TRH may cause nausea, vomiting and some patients experience an urge to urinate.
[citation needed] Rarely, TRH may cause blood vessel constriction leading to hemorrhage in patients with pre-existing pituitary tumors.