Thyroid hormones biological actions

Most of the physiologic activity of thyroid hormones is from the actions of T3. T4 can be thought of primarily as a prohormone. Eighty percent of needed T3 is derived from the conversion of T4 to T3 in peripheral tissue under the influence of tissue deiodinases. These deiodinases allow end organs to produce the amount of T3 needed to control local metabolic functions. These enzymes also catabolize T3 and T4 to biologically inactive metabolites. Thyroid hormones bind to intracellular receptors and regulate the transcription of various genes. 

Among the active compounds were xanthurenic and 3-hydroxyanthra-nilic acids, whereas tryptophan, kynurenine, and kynurenic, anthranilic, and nicotinic acids were unable to cause deiodination. These findings support, according to the authors, the view that the deiodination of the thyroid hormone may be closely associated with its biological action. 

Dioxins and dioxin-like PCBs induce a broad spectrum of toxic responses, such as loss of body mass, hepatotox-icity, immunotoxicity, epidermal changes, embryotoxicity and carcinogenicity. In addition, they can act as endocrine disrupters, thereby mimicking or interfering with the action of endogenous hormones and other molecules of the endocrine system. Besides their interference with the reproductive system, there are indications of their role in thyroid hormone (TH) metabolism and function. The toxic and biological effects of these PHAHs depend on several factors, such as doses, route of administration, species, age, strain and sex of the animals (Safe, 1986). 

Tata JR. Inhibition of the biological action of thyroid hormones by actinomycin D and puromycin. Nature 1963 197 1167-1168. 

Oppenheimer JH, Samuels HH. The nuclear receptor-triiodothyronine complex relationship to thyroid hormone distribution, metabolism and biological action. In Oppenheimer JA, Samuels HH, eds. Molecular Basis of Thyroid Hormone Action. New York Academic Press, 1983, pp. 1-34. 

Effects of ID in adult rats Feeding adult rats a diet with a low iodine content (LID) (0.02-0.06 pg I / g) results in undetectable circulating T4 levels, increased TSH and unaltered T3 concentrations, the same changes described for inhabitants of areas with severe ID. T4 decreases in all tissues studied so far (liver, brain, heart, and skeletal muscle) to the same degree as in plasma In contrast, T3 does not remain normal in tissues, as it does in plasma. T3 levels decrease, although not as markedly as T4 T3 in nuclei from liver and brain decrease 2- 2.5-fold with respect to nuclear T3 in rats on the I-supplemented LID (LID+I) All biological end-points of thyroid hormone action... 

---