Role of T3 in fasted man

The Role of Leptin in Maintaining Plasma Glucose During Starvation

For 20 years it has been known that concentrations of leptin, a hormone produced by the white adipose tissue (WAT) largely in proportion to body fat, drops precipitously with starvation, particularly in lean humans and animals. The role of leptin to suppress the thyroid and reproductive axes during a prolonged fast has been well defined; however, the impact of leptin on metabolic regulation has been incompletely understood. However emerging evidence suggests that, in starvation, hypoleptinemia increases activity of the hypothalamic-pituitary-adrenal axis, promoting WAT lipolysis, increasing hepatic acetyl-CoA concentrations, and maintaining euglycemia. In addition, leptin may be largely responsible for mediating a shift from a reliance upon glucose metabolism (absorption and glycogenolysis) to fat metabolism (lipolysis increasing gluconeogenesis) which preserves substrates for the brain, heart, and other critical organs. In this way a leptin-mediated glucose-fatty acid cycle appears to maintain glycemia and permit survival in starvation.

Ovarian hormone effects on activity, glucoregulation and thyroid hormones in the rat

Ovarian hormonal influences on the range of physiological and behavioral variables which combine to affect overall energy balance are poorly delineated. In the present study 4 groups of virgin, female rats (intact, ovariectomized, ovariectomized with estrogen replacement and ovariectomized with estrogen plus progesterone) were allowed access to running wheels and activity; food intake and weight gain were measured initially under food restricted, then under ad lib conditions. Serum insulin, glucose, thyroxine (T4) and triiodothyronine (T3) were determined on trunk blood samples obtained at the end of the experiment. Ovariectomy resulted in an increased rate of weight gain through reduced activity and T3 but food intake was unchanged. Insulin levels were greatly reduced. Estrogen replacement restored activity to the intact group's level and normalized weight gain. Insulin and T3 were also raised to control levels but T4 was reduced as was serum glucose. Estrogen plus progesterone replacement reduced weight gain markedly and increased T3 with normal T4. Despite the lower body weight this group was hyperglycemic and hyperinsulinemic suggesting insulin resistance. The results have important implications for the glucoregulatory and energy balance perturbations of ovarian hormone fluctuations and focus particularly on progesterone.

Comparative study of pituitary-thyroid hormone economy in fasting and hypothyroid rats

Starvation in laboratory rodents results in significant alterations in thyroid hormone economy characterized by decreased circulating levels of thyroxine (T4) and 3,5,3'-triiodothyronine (T3) and a decline in serum thyrotropin (TSH) concentration. To investigate this apparent paradox, we have compared in fasted and hypothyroid animals the intracellular parameters mediating thyroid hormone action in the anterior pituitary gland. In vitro saturation analysis combined with quantitation of nuclear T3 content by radioimmunoassay allowed for characterization of pituitary nuclear T3 receptors and estimation of the endogenous fractional receptor occupancy. In rats, thyroidectomized 4 wk earlier, the 10-fold increase in serum TSH levels and decline in peripheral thyroid hormone concentrations were accompanied by a 61% decrease in pituitary nuclear T3 content and a marked decline in fractional T3 receptor occupancy as compared with control animals. In euthyroid animals subjected to short-term starvation (72 h), serum T3, T4, and TSH levels declined by 52, 43, and 48%, respectively. Despite these marked decreases in circulating thyroid hormone levels, pituitary nuclear T3 content in fasted rats declined by only 15% (P less than 0.05) relative to control levels. This modest decline in nuclear T3 content, combined with a 23% decrease in total T3 receptor number, resulted in an estimated fractional receptor occupancy in fasted animals which was equal to or greater than that noted in controls. The effects of fasting and hypothyroidism on the pituitary were further investigated by quantifying low Michaelis constant (Km) T4 5'-deiodinase activity in the crude cytosol fraction of pituitary homogenates. In thyroidectomized animals, maximum velocity was increased ninefold, whereas fasting resulted in a 37% decrease (P less than 0.025) in this parameter compared with controls. Km values were similar in all experimental groups (4.7 +/- 0.6 nM). These results demonstrate that, despite significant reductions in circulating thyroid hormone concentrations and pituitary T4 5'-deiodinase activity, nuclear T3 levels are maintained at relatively normal levels in the pituitary of the fasted animal and fractional T3 receptor occupancy may actually increase. These findings are in marked contrast to those noted in thyroidectomized animals and suggest that the suppression of TSH secretion accompanying starvation in the rat is mediated, at least in part, by local pituitary mechanisms that serve to maintain and possibly enhance nuclear T3 receptor occupancy.

Thyroid hormone action on intermediary metabolism. Part III. Protein metabolism in hyper- and hypothyroidism

In their physiological concentrations, thyroid hormones stimulate the synthesis as well as the degradation of proteins, whereas in supraphysiological doses protein catabolism predominates. In hyperthyroidism skeletal muscle protein stores suffer depletion which is reflected by an increased urinary N- and methylhistidine -excretion. Due to the enhanced skeletal muscle amino acid release, the plasma concentration of glucoplastic amino acids are often enhanced, contributing by means of an elevated substrate supply to the increased hepatic gluconeogenesis. Thyroid hormone excess induces cardiac hypertrophy which is in direct contrast to the hypotroph skeletal muscle in hyperthyroid patients. Thyroid hormones stimulate a series of intracellular and secretory proteins in the liver, although in hyperthyroid liver alcohol dehydrogenase and the enzymes of histidine and tryptophan metabolism show reduced activities. The stimulatory effect is due to thyroid hormone-induced increase in the protein synthesis at a pretranslational level and is supported experimentally for malic enzyme, alpha 2u-globulin and albumin by the measurement of their specific messenger RNA activities. Thyroid hormone action at the cellular level is reflected by a generalized increase in total cellular RNA with a selective increase or decrease in a small population of specific mRNA. The activities of protein catabolizing lysosomal enzymes are stimulated by thyroid hormones; up to now effects of T3 on the degradation of specific enzymes have not been reported. Serum total protein concentration is slightly reduced or even unchanged in hyperthyroidism. The thyroid hormone-induced increase in the turnover of total body protein is part of the hypermetabolism observed in hyperthyroidism.