Functions of thyroid hormones
The Functions of thyroid hormones can be roughly divided into two sections-Morphological (growth and development) and Functional (metabolic) effects.
Morphological functions of thyroid hormones
Thyroid hormones are required for normal growth and skeletal maturation. They potentiate the effects of GH and act in synergy with GH in promoting normal growth of animals. TH is required for maturational changes in vertebrates and helps the growth of the young birds and mammals. Even though GH stimulates growth, for differentiation thyroid hormones are essential.
Thyroid hormone helps in metamorphosis in amphibians. Thyroxine administration causes differentiation of tadpoles into frogs.
Thyroxine controls the growth and eruption of the teeth and also horns in sheep and antlers in deer. Hypothyroidism severally retards the eruption of permanent teeth.
Thyroid hormone is also necessary for the normal development of nervous tissue in fetus and neonates. Thyroid deficiency in young leads to retardation of mental activity.
Thyroid hormone acts synergistically with sex steroids to function in growth and maintenance of comb in roosters and moulting of feathers in birds.
In mammals, renewal of epidermis and hair follicles are affected by thyroxine. Thyroidectomized cattle and sheep have thinner hairs, which are coarse and brittle. In growing lamb, thyroxine deficiency may severally impair the quality of the adult fleece.
In human beings and sometimes in dogs, hypothyroidism causes subcutaneous edema due to accumulation of mucopolysaccharide rich material under the skin. This condition is called as myxedema. Myxedema and alopecia have also been observed in calves and pigs born to iodine‑deficient mothers.
TH (Thyroid hormone) helps the normal process of spermatogenesis, folliculogenis, and maturation of follicle, ovulation and maintenance of a healthy pregnant state.
Reproductive failure is often a major sign of thyroid deficiency. Iodine deficiency causes birth of excessive numbers of weak or dead young. Abortion, stillbirth, and the live birth of weak young are the major problems of hypothyroidism. Less severe deficiency will result in delayed puberty, irregular estrous, anestrus, and reduced fertility in the females and decreased testicular growth, impaired spermatogenesis and lowered libido in males. In rams, a seasonal reduction in semen quality has been associated with hypothyroidism.
The accessory reproductive glands, which are most sensitive to the effect of the thyroid hormone, are the mammary gland.
Thyroxine is a powerful galactopoietic agent;
Thyromimetic agents have been tried as a means of increasing milk production. Iodinated casein fed to cattle increased milk production by 10-30 %.
Metabolic functions of thyroid hormones
Thyroid hormone increases the metabolic rate and stimulates growthin young ones. It increases the rate of O2 consumption (up to 200%) and heat production which is known is calorigenic effect.
Thyroid activity increases following low environmental temperature, thus involved in thermo-regulation to increase internal heat production by non-shivering thermogenesis
Thyroid hormone causes increased metabolic activities in almost all tissues of body except the brain, retina, spleen, testes and lungs.
Thyroid hormone increases the size and number of mitochondria to increase the ATP production. It increases the activities of the respiratory enzymes and enzymes involved with glucose oxidation and gluconeogenis. It stimulates Na+‑K+ ATPase activities, favours cations transport and thus stimulates mitochondrial O2 utilization.
BMR is greatly increased by Thyroid hormone .
Thyroid hormone increases cardiac output ensuring sufficient O2 delivery to the tissues. It potentiates the actions of β ‑adrenergic receptors to catecholamines, thus stimulates the rate and force of contraction of the heart.
Thyroid hormone increases glucose absorption from the intestine and facilitates insulin-mediated glucose absorption by the cells and produces glycogenolysis.
Thyroid hormone increases protein synthesis by stimulating mRNA synthesis and the rate of formation of proteins (enzymes) by ribosomes.
It activates catabolic pathway and inhibits anabolic pathway. It enhances the actions of catecholamines which include non‑shivering thermogenisis, lipolytic, glycogenolytic, gluconeogenic and insulin secretion .
Thyroid hormone potentiates the stimulatory effects of other hormones on metabolism by increasing the uptake of glucose by cells, enhanced glycolysis, gluconeogenesis, and absorption from gastro-intestinal tract lipolysis, ketogenesis and proteolysis.
Thyroid hormone lowers blood cholesterol level due to increased removal by liver, mobilizes fat for energy, depletes fat stores of the body, increases free fatty acid levels in blood and enhances oxidation of fatty acids. One of the major metabolic results of thyroid deficiency is a marked increase in the serum cholesterol, phospholipids, triglycerides and decreased free fatty acid levels.
The thyroid hormones influences nervous functions at all levels. Injection of thyroxine causes increased spontaneous electrical activity in the brain.
Thyroxine enhances the speed and amplitude of reflexes, wakefulness, alertness to various stimuli, awareness of hunger, memory and learning capacity. It regulates creatine phosphate production and storage in the muscle tissue. Thyroid deficiency is a major factor for obesity and increased body weight.