Protirelin

Protirelin is a synthetic tripeptide (pyroglutamyl-histidyl-prolinamide) identical in structure to the naturally occurring thyrotropin-releasing hormone (TRH) produced by the hypothalamus. TRH is one of the simplest known peptide hormones and plays a central role in the hypothalamic-pituitary-thyroid axis by stimulating the release of thyroid-stimulating hormone (TSH) and prolactin from the anterior pituitary gland.

The TRH stimulation test using protirelin was historically one of the most important diagnostic tests in clinical endocrinology. In this test, a bolus injection of protirelin (200-500 mcg IV) is administered, and serum TSH levels are measured at baseline and at 15, 30, and 60 minutes. The pattern of TSH response provides valuable diagnostic information: an exaggerated response suggests primary hypothyroidism, a blunted or absent response suggests secondary (pituitary) hypothyroidism or TSH-suppressive conditions, and a delayed response suggests hypothalamic dysfunction.

The TRH stimulation test was particularly useful for distinguishing between hypothalamic and pituitary causes of central hypothyroidism, evaluating subtle TSH secretory abnormalities, and as a confirmatory test for thyrotoxicosis before the development of sensitive TSH assays. It was also used in psychiatric research to study the relationship between thyroid axis function and mood disorders, as blunted TRH responses have been associated with major depression.

With the development of highly sensitive third-generation TSH assays capable of accurately measuring very low TSH levels, the clinical need for the TRH stimulation test has diminished substantially. Modern TSH assays can reliably detect suppressed TSH in hyperthyroidism and elevated TSH in hypothyroidism without the need for dynamic testing. Consequently, protirelin has been withdrawn from the US market, though it remains available in some countries for specialized diagnostic applications.