1) To diagnose central/secondary hypothyroidism
The hypothalamus secretes the thyrotropin-releasing hormone (TRH) that directs the synthesis and secretion of TSH from the pituitary gland. 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.
The TRH test involves administration of a small amount of TRH intravenously, followed by measuring levels of TSH at several subsequent times, taken from a peripheral vein. Patients with compromised hypothalamic-pituitary axis function (secondary/central hypothyroidism) may exhibit a delayed, blunted, or absent response to TRH administration.
2) To diagnose idiopathic isolated central hypothyroidism
A central hypothyroidism is suspected, but no lesion is found by MRI, no other pituitary hormone insufficiency is detected, no intake of any drug affecting the hypothalamo-pituitary-thyroid axis and no presence of critical systemic illness. Thus, the unusual diagnosis of idiopathic (no cause found) isolated central hypothyroidism can be made. TSH response to TRH stimulation test confirms the diagnosis.
3) To help diagnose of primary hypothyroidism (including Hashimoto's)
The measurement of TSH should be elevated in cases of overt hypothyroidism, and, to a lesser extent, in subclinical hypothyroidism as well. But routine TSH test can be terribly misleading. As I have already said, TSH values may appear to be normal even though one has a low thyroid. If hypothyroid symptoms are present (with positive thyroid antibodies or in their absence), but TSH test are normal, Drs. Raphael Kellman and John C. Lowe rely on the TRH stimulation test to assess possible primary hypothyroidism.
Patients with normal function of the hypothalamic-pituitary axis respond by increasing the levels of TSH following TRH injection, indicating primary hypothyroidism.
According to Dr. Kellman, "the physician measures the patient's TSH level, gives an injection of TRH, then draws blood 25 minutes later and remeasures the TSH. If the first TSH level is normal and the second TSH level is high - above 10 mIU/L - it tells us the patient's thyroid is underactive. TSH reading of 15 is suspicious, while 20 strongly points to hypothyroidism."
Dr. Kellman has tested over 13,000 patients and in a large percentage, he detected what the patients themselves suspected: hypothyroidism. Dr. Kellman states that, "of the patients I've seen with three or more typical symptoms of underactive thyroid but who have tested "normal" in standard tests, 35-40% have underactive thyroids based on the TRH test… The TRH stimulation test is a challenge test that helps us look beneath the tip of the iceberg."