Basal Metabolic Rate and Thyroid Hormones

3,5-T2-an Endogenous Thyroid Hormone Metabolite as Promising Lead Substance in Anti-Steatotic Drug Development?

Thyroid hormones, their metabolites, and synthetic analogues are potential anti-steatotic drug candidates considering that subclinical and manifest hypothyroidism is associated with hepatic lipid accumulation, non-alcoholic fatty liver disease, and its pandemic sequelae. Thyromimetically active compounds stimulate hepatic lipogenesis, fatty acid beta-oxidation, cholesterol metabolism, and metabolic pathways of glucose homeostasis. Many of these effects are mediated by T3 receptor β1-dependent modulation of transcription. However, rapid non-canonical mitochondrial effects have also been reported, especially for the metabolite 3,5-diiodothyronine (3,5-T2), which does not elicit the full spectrum of “thyromimetic” actions inherent to T3. Most preclinical studies in rodent models of obesity and first human clinical trials are promising with respect to the antisteatotic hepatic effects, but potent agents exhibit unwanted thyromimetic effects on the heart and/or suppress feedback regulation of the hypothalamus-pituitary-thyroid-periphery axis and the fine-tuned thyroid hormone system. This narrative review focuses on 3,5-T2 effects on hepatic lipid and glucose metabolism and (non-)canonical mechanisms of action including its mitochondrial targets. Various high fat diet animal models with distinct thyroid hormone status indicate species- and dose-dependent efficiency of 3,5-T2 and its synthetic analogue TRC150094. No convincing evidence has been presented for their clinical use in the prevention or treatment of obesity and related metabolic conditions.

Experimental hyperthyroidism IV. Myocardial muscle mechanics and oxygen consumption in euand hyperthyroidism

Myocardial mechanics and oxygen consumption were studied in right ventricular papillary muscles taken from cats pretreated with cristalline L-thyroxine (1 mg/kg/day, i.p.) 8-18 days prior to the examination. Isotonic afterloaded and isometric contractions were employed. Oxygen consumption was determined polarographically. Data obtained were compared with control studies on papillary muscles taken from euthyroid cats. In isotonic afterloaded contractions the extent of shortening was nearly identical in both groups. However, maximum rate of isometric tension development and velocity of isotonic shortening were considerably increased in hyperthyroid myocardium. Myocardial oxygen consumption was significantly increased in hyperthyroidism, primarily due to an increased maximum rate of isometric tension development and-to a lesser extent-to increased isotonic contraction velocity. In isometric contractions maximum tension development (preload near Lmax) was similar in both groups. However, maximum rate of isometric tension development was markedly increased in hyperthyroidism. A close and linear relationship was found between maximum rate of isometric tension development (isometric contraction) and myocardial oxygen consumption. The results demonstrate increases of velocity factors of myocardial performance in experimental hyperthyroidism. Myocardial oxygen consumption is significantly increased. This increase in oxygen consumption quantitatively has its mechanical equivalent in increased isometric contraction velocity and, to a small amount, in increased isotonic contraction velocity.

Thyroid activity and growth rate in rapidly growing lambs

1. This paper reports a study of the relationship between thyroid gland activity and rate of growth of lambs held in a controlled environment with feeding ad libitum.2. Thyroid activity was assessed by the rate of release of (131)I from the thyroid gland over a period of 1-2 weeks following injection. Growth rates were simultaneously recorded on a total of thirty-one lambs during forty-six measurements of thyroid activity.3. The results demonstrated a highly significant curvilinear correlation between growth rate and the rate constant for thyroid (131)I release (P < 0.001).4. Surgically thyroidectomized animals were demonstrated to have a much reduced rate of growth.5. It was concluded that thyroid activities considerably above or below the optimum have an adverse effect on growth rate.