The simultaneous occurrence a low thyroid hormone level and a normal or low serum level of thyroid stimulating hormone (TSH) in a patient with hypothyroidism establishes the diagnosis of secondary hypothyroidism. The administration of thyrotropin releasing hormone (TRH), which is normally synthesized and released by the hypothalamus, enables the physician to distinguish between pituitary and hypothalamic causes of hypothyroidism.
TRH will normally cause the release of TSH from the pituitary gland. When the hypothalamus does not synthesize or release TRH, the administration of exogenous TRH will result in prompt rise of serum TSH. To date, the cases reported of pituitary hypothyroidism. Hypothalamic hypothyroidism, on the other hand, may either be idiopathic or result from demonstrable hypothalamic disease.