Thyrotrophin releasing hormone--TSH.

The major determinant of serum TSH and its response to TRH is the interaction of TRH and the thyroid hormones (principally T3) at the level of the thyrotroph. Small changes in thyroid hormone levels within the normal range can produce marked alteration in thyrotroph sensitivity to TRH. Elevated serum TSH is of great diagnostic value in suspected primary hypothyroidism. The demonstration of a normal TSH response to TRH is valuable in excluding the diagnosis of hyperthyroidism, whereas a suppressed response is of lesser value in diagnosis, although characteristic of the hyperthyroid state. TSH responses to TRH are of limited value in the differentiation of hypothalamic from pituitary lesions, and cannot always be correlated with clinical or biochemical thyroid status.