Biochemical Actions of Thyroid Hormones

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Abstract
The relatively simple structure of thyroxine makes it an inviting molecule for attempts at understanding hormone action. This promise of simplicity, however, is not apparently borne out by the multiplicity of thyroid hormone actions. Variations of thyroid hormone concentrations in vivo alter respiration, temperature regulation, growth, development, response to other hormones, nerve function, and the metabolism of proteins, fats, carbohydrates, nucleic acids, vitamins, anions and cations. In vitro the thyroid hormones in-hibit the activity of a number of purified enzymes and change the function and structure of mitochondria; the actions on mitochondria are duplicated by in vivo hormonal effects. If it is accepted that hormones act by reacting chemically at a locus on their target molecules, the structure of thyroxine suggests that the variety of physiological, chemical and physical manifestations can be attributed to a few types of interactions at the molecular level. Alternatively, the analogues of thyroxine may each have a different and specific reactivity that produces a different physiological result, and the conversion of one analogue to another accounts for the variety of hormone effects. 611 references.

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