The role of thyroid hormones in the regulation of metabolism.

By….S.Hari Baskar

THYROID GLAND

• Thyroid gland synthesizes two major of hormones: Thyroxine and Triiodothyronine

Functions of iodine containing hormones

Necessary for normal growth, differentiation, sexual and mental development

Regulate the rate of metabolism

• Required for Growth Hormone action

• Increases mitochondrial oxidative phosphorylation

• Increases activity of adrenal medulla

• Induces enzyme synthesis

• Result:Stimulation of growth of tissues and increased metabolic rate along with Increased heat production

Anabolic Actions of Thyroid Hormone

• About 1/3rd of all ATP in the body is used by the Na+/K+ ATPase

• T3 increases the synthesis of Na+/K+ pumps, markedly increasing ATP consumption.

• T3 also acts on mitochondria to increase ATP synthesis

• The resulting increased metabolic rate increases thermogenesis (heat production).

One Major Target Gene of T3: The Na+/K+ ATPase Pump

Calorigenic action

• Stimulates heat production in body secondary to increased oxygen consumption which increases BMR.

• Exception:- Adult brain ,Testis, Lymph nodes, Ovary, Uterus, Anterior pituitary.

•In physiological concentration it is anabolic causing a positive nitrogen balance

Effect of thyroxine and triiodthyronine on the protein

metabolism:

• In the increased concentration it is catabolic causing negative nitrogen balance

• Thus muscle weakness and cretinuria are common manifestations of a Hyperthyroid patient.

Effect on carbohydrate metabolism

T3 and T4 leads to overall increase in enzymes causing

• Increased glucose absorption from GIT.

• Acceleration of Glucose metabolism-Rapid uptake by cells, Enhanced glycolysis and increased insulin secretion and its effects on carbohydrate metabolism.

• Thus hyperthyroidism causes Diabetes mellitus

Effect on Fat metabolism• Mobilization of fat from the

Adipose tissue.• Increase in level of free

fatty acid • Enhanced oxidation of free

fatty acids by cells of body.• Decrease in Quantity of

cholesterol, Phospolipids and TAG in plasma.

• All these factors leads to incresed loss of weight.

• Decrease in plasma cholesterol is due to increased excretion in BILE.

• P.Chloesterol level drops even before increase in BMR, Showing that the action is independent of oxygen consumption.

• So, Hypothyroidism is associated with elevated plasma cholestrol levels, which can be reversed by thyroid administration.

Vitamin metabolism• Thyroid hormones increase the

quantity of enzymes• Vitamins are essential for some

parts of enzymes and co-enzymes

• Thus, Thyroid hormones results in increased need of Vitamins.

• This is the reason for vitamin deficiency in Hyperthyroidism.

E.g. Excess thyroxin causes depletion of niacin leading to decrease in NADH concentration .

On Water and Electrolyte balance

• Has a significant role in regulation of water and electrolyte balance.

• Impairment of thyroid function results in accumulation of Water and electrolytes which can be reversed by administration of these hormones.

Permissive actions……

• T3 and T4 potentiate the stimulatory effects of Epinephrine, Nor-Epinephrine, Glucagon, Cortisol and Growth hormone on Gluconeogenesis, Lipolysis, Ketogenesis and Proteolysis of liable protein pool.

On Sleep………• Due to exhausting effect

of thyroid hormone on the musculature an CNS, the hyperthyroid subject has a feeling of constant tiredness, But because of the excitable effects of thyroid hormone on the synapses, it is difficult to sleep.