A TRH test may be indicated if secondary hypothyroidism

Hypothyroidism

Hypothyroidism is a condition in which the thyroid gland does not make enough thyroid hormone.Iodine deficiency is the most common cause of hypothyroidism worldwide but it can be caused by other causes such as several conditions of the thyroid gland or, less commonly, the pituitary gland or...

 is suspected. Some patients may have low levels of circulating thyroid hormones and secondary hypothyroidism as a result of damage to the hypothalamic or pituitary control mechanisms that regulate thyroid function. The hypothalamus secretes the thyrotropin-releasing hormone

Thyrotropin-releasing hormone

Thyrotropin-releasing hormone , also called thyrotropin-releasing factor , thyroliberin or protirelin, is a tropic tripeptide hormone that stimulates the release of thyroid-stimulating hormone and prolactin by the anterior pituitary...

 that directs the synthesis and secretion of TSH

TSH

TSH may refer to:* Tanzanian shilling, the currency of Tanzania* Thyroid-stimulating hormone, a hormone synthesized and secreted by thyrotrope cells* Tshiuetin Rail Transportation, a Canadian railway between Labrador and Quebec...

 from the pituitary gland

Pituitary gland

In vertebrate anatomy the pituitary gland, or hypophysis, is an endocrine gland about the size of a pea and weighing 0.5 g , in humans. It is a protrusion off the bottom of the hypothalamus at the base of the brain, and rests in a small, bony cavity covered by a dural fold...

. If these normal regulatory mechanisms are interrupted, the pituitary may not be able to produce appropriate levels of TSH and levels of thyroid hormones may decline, although the TSH remains appropriately normal.

Process and interpretation

The TRH test involves administration of a small amount of TRH intravenously, following which levels of TSH will be measured at several subsequent time points using samples of blood taken from a peripheral vein.

• Patients with normal function of the hypothalamic-pituitary axis (HPA) respond by increasing the levels of TSH following TRH injection.
• Patients with compromised HPA function may exhibit a delayed, blunted, or absent response to TRH administration.

Side effects and risks

TRH may cause nausea, vomiting and some patients experience an urge to urinate.

Rarely, TRH may cause blood vessel constriction leading to hemorrhage in patients with pre-existing pituitary tumors. Accordingly, patients should be advised about the risks, albeit rare, of TRH testing.