Immunoassay Studies of Thyrotropin in Rat Pituitary Glands and Serum

Abstract

A sensitive and specific radioimmunoassay for rat thyrotropin (TSH) has been developed which utilizes 125I -mouse thyro-tropic tumor TSH, antibovine TSH serum and USP bovine TSH as a standard. TSH in serum and pituitary homogenates was found in frac -tions corresponding to those previously shown to contain biological activity after gel filtration on Sephadex G-200. Serum TSH in normal rats after decapitation ranged from 15 to 66 [mu]U/ml. During pento-barbital anesthesia, serum TSH was significantly lower (10 [long dash] 37 [mu]U/ml). Mean normal rat pituitary TSH content was 44 mU/gland. After 50 ug thyroxine (T4) every other day for 14 days, serum TSH became un-detectable (< 6 [mu]U/ml) in 5 of 7 rats, and pituitary content fell to 25% of normal. Chronic propylthiouracil (PTU) treatment resulted in a progressive increase in serum TSH over a 12-week period. Pituitary TSH content, however, was below normal after 3 weeks of PTU, but rose progressively thereafter to exceed normal at 12 weeks. Following intravenous administration of thyroxine to PTU-treated rats, TSH fell exponentially with a mean t1/2 of disappearance of 29 min. Twenty-four hr. after T4, serum TSH fell to normal, while pituitary content increased 25% over control. These results show that pituitary TSH synthesis exceeds release after chronic thyroid hormone deprivation and that thyroxine administration acutely inhibits TSH synthesis as well as release.