The use of intramuscularly administered methyl-TRH to evaluate the pituitary-thyroid axis

The present study was carried out to evaluate the effectiveness of intramuscular administration of methyl-TRH, a potent analogue of thyrotropin-releasing hormone, for assessing pituitary reserve of TSH and prolactin and for distinguishing euthyroid, hypothyroid and hyperthyroid individuals. Serum samples were taken for 24 hours after intramuscular injection of methyl-TRH, 200 microgram, in 19 euthyroid subjects, 9 hypothyroid men and 9 hyperthyroid men. The mean serum prolactin and TSH concentrations were significantly elevated over baseline levels at 30 min in the euthyroid individuals and remained elevated for 3 to 4 hours. The serum TSH, T3 and T4 responses after intramuscular methyl-TRH in euthyroid subjects were clearly distinguishable from those of hyperthyroid and hypothyroid patients. Significant elevation of the serum T3 and T4 concentrations at 24 hours after intramuscular injection of methyl-TRH shows the sustained effect of this TRH analogue in euthyroid subjects.