Physiology lecture on endocrine system physiology explains the functions of thyroid hormones.

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Thyroid hormones act on virtually every cell on the body.
In general, the basic function of thyroid hormones is to increase the metabolic rate, and oxygen consumption

For this, thyroid hormones increase the production as well as utilization of ATP. They increase the oxidative phosphorylation of ADP to ATP in mitochondria by increasing the number of mitochondria and the production of cytochrome oxidases which are enzymes involved in respiratory chain. Utilization of ATP increases because of increase in permeability of membrane to Na+ and K+ions. The change in intracellular concentration of ions, causes increase in the activity of Na+K+ATPase which is also directly increased by thyroid hormones. Also, Thyroid hormones speed up the synthesis as well as degradation metabolic cycles of carbohydrates, fats and protein. Thyroid hormones also increase the synthesis of uncoupler protein which uncouples oxidation and phosphorylation reactions i.e proton movement is uncoupled with the synthesis of ATP. This leads to heat generation without even ATP production and utilization.

In Nervous system, thyroid hormones are essential for differentiation and maturation of neurons. For normal development of nervous system, thyroid hormones make the switch from multiplicationof neurons to differentiation.

In later life also, thyroid hormones maintain the mental alertness.

Body growth: Thyroid hormones are also essential for normal body growth because thyroid hormones are required for normal production and action of growth hormone and insulin like growth factor1. They also have a direct effect on bone growth and promote closure of epiphysis.

Cardiovascular system: Apart from having an indirect effect on heart by virtue of increased metabolic activity, thyroid hormones also have a permissive action on heart. They increase the responsiveness of heart to catecholamines by increasing the expression of their receptors i.e beta adrenergic receptors. Thyroid hormones also increases the expression of proteins involved in excitation contraction coupling in heart i.e SERCA, sodium potassium ATPase and others.

Other systems: Thyroid hormones are essential for normal function of other systems as well for e.g. menstrual cycles, lactation, development of gonads, normal GI function, hematopoiesis.

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