Thyroxine (T4) and Triiodothyronin (T3) are the two thyroid hormones. These two hormones have the same functions. Although T3 is more potent, T4 is considered as the chief hormone of thyroid, because the latter is secreted many times more than the former quantitatively. For our discussion about the function of hormone, the actions of T4 and T3 in the body are as follows:--
BMR and calorigenesis
T3 and T4 increase the BMR by stimulating oxidation of foodstuff in the tissues and therefore these hormones are also called calorigenic hormones. During cold, the secretion of thyroid hormones is increased, which by stimulating calorigenesis help in thermoregulation and avoid fall of body temperature.
Metabolism of Carbohydrate
Another function of thyroid hormone is to raise the blood sugar level by increasing food intake, absorption of glucose from intestine, glycogenolysis, neoglucogenesis etc. These hormones also increase the oxidation of glucose in tissues as well.
Metabolism of Protein
Under normal conditions that means, when the blood levels of T4 and T3 are within normal range, these hormones help in protein synthesis but when secreted in excess, they cause protein breakdown.
Metabolism of Fat
T3 and T4 stimulate the catabolism of fat by activating the enzyme lipase present in the tissues.
Growth and differentiation
Another function of thyroid hormone is the growth and differentiation. Thyroid hormones help in growth by acting jointly with STH. STH directly acts on tissues to stimulate their growth. T4 and T3 stimulate differentiation of tissues, which indirectly helps in growth by facilitating the action of STH. Requirement of thyroid hormones in metamorphosis of amphibians and molting of reptiles, birds etc., are good examples of developmental effect of these hormones.
Action on nervous system
These hormones help in the development of nervous system and mental functions as evidenced by the fact that in deficiency of these hormones, the subject becomes mentally retarded.
Action on circulatory system
As the function of thyroid hormone its action on circulatory system is also to be noted. Thyroid hormones increase absorption of vitamin B12 from the intestine, which is required for synthesis of hemoglobin and thereby erythropoiesis (i.e.,formation of RBC). T4 and T3 increase the activity of heart.
Action on reproductive system
These hormones help in the growth, development and function of reproductive system. This is most likely due to the stimulatory effect of T4 and T3 on pituitary GTH secretion.
Action on digestive system
Function of thyroid hormone on digestive system is promote hunger, appetite, movement of gastro-intestinal tract and absorption of various materials from the intestine
Hypo- and hyper-secretion of T4 and T3