EFFECTS OF TRI-IODOTHYRONINE, THYROXINE AND ISOPROPYL-DI-IODOTHYRONINE ON THYROID-STIMULATING HORMONE IN SERUM AND PITUITARY GLAND AND ON PITUITARY CONCENTRATIONS OF PROLACTIN, GROWTH HORMONE, LUTEINIZING HORMONE AND FOLLICLE-STIMULATING HORMONE IN HYPOTHYROID RATS

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Abstract
Replacement of the 3′-halogen of the tri-iodothyronine (T3) molecule by a propyl-group produces a thyromimetic analogue, 3′-isopropyl-3,5-di-iodo-l-thyronine (T2iPr), with high biological potency. A serum thyroid-stimulating hormone (TSH) suppression test with one single intraperitoneal injection of 3 or 30 nm-T3 or T2iPr or with 30 or 300 nm-thyroxine (T4) per kg body weight was performed on 56 adult male Lewis rats which were maintained for 3 weeks on an iodine-deficient diet containing 0·2% 6n-propyl-2-thiouracil (PTU). Blood was withdrawn from each rat by cardiac puncture 24 h before and 3, 7, 24 and 48 h after application of the iodothyronines. Raised serum levels of TSH, due to the treatment with PTU, were significantly reduced within 3 h of treatment with 30 nm-T3, 300 nm-T4, 3 or 30 nm-T2iPr and they remained low throughout the observation period. Treatment with 3 nm-T3 or 30 nm-T4 per kg body weight was less effective. Pituitary concentrations of growth hormone, TSH, prolactin and FSH were significantly reduced by the treatment with PTU. There was also a slight, but insignificant reduction of pituitary concentrations of LH. Treatment with T3, T4 or T2iPr stimulated the reaccumulation of growth hormone, TSH, prolactin, LH and FSH in the pituitary gland.

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