Response of thyroid hormones in peripheral and thyroid venous blood to TRH administration in man

Thyroid vein and cubital vein samples were collected simultaneously in 6 moderately hypercalcemic patients and 1 eucalcemic hypothyroid patient, and thyroid hormones were measured in the serum in the basal state, as well as 30 and 60 min after intravenous administration of TRH. No gradient was detectable between peripheral and thyroid blood in the case of T4, and no significant changes were observed following TRH. The levels of T3 were higher in the thyroid venous effluent than in the periphery and a marked increase occurred following TRH. Serum thyroglobulin also increased following TRH, but there was no peripheral vs. thyroid gradient. Calcitonin demonstrated a marked positive gradient in the thyroid vein compared to the periphery, but no change was observed following TRH. It is concluded that the patterns of response of individual thyroid hormones reflect differences in their secretion and, specifically, that intrathyroid conversion of T4 to T3 occurs during the thyroid hormone secretion.