M.Prasad Naidu MSc Medical Biochemistry, Ph.D,. Thyroid hormones M.Prasad Naidu MSc Medical Biochemistry, Ph.D,.

Thyroid gland produces two principal hormones … thyroxine & tri iodo thyronine which regulate the metabolic rate of the body. Iodine is essential for the synthesis of thyroid hormones More than half of the body’s total content is found in the thyroid gland

Hypothalamo pituitary axis The hypothalamo-pituitary axis is a classical negative feedback regulatory mechanism in which secretion of TSH is modulated by thyroid hormones. Release of TSH from the pituitary gland is stimulated by thyrotropin releasing hormone (TRH) from the hypothalamus.

Hypothalamo pituitary axis A small increase in T3 and T4 produces a diminished TSH response to TRH at the pituitary level. T3 and T4 act at the hypothalamic level by inhibiting mRNA for TRH synthesis. Only unbound fractions of hormone are metabolically active and only this free hormone has an inhibitory effect on the secretory activity of the thyroid. dopamine physiologically inhibits TSH secretion glucocorticoids have been shown to dull the response of the pituitary to TRH oestrogens increase the sensitivity of thyrotrophs to TRH

Mechanism of thyroid hormone receptor action

Actions of thyroid hormones Brain----growth&development of nervous system Bone&tissue growth– linear growth & maturation of bones CVS-- increased contractility,heart rate &cardiac output GUT—increased absorption of nutrients, increased motility Liver -increased gluconeogenesis&glyco genolysis Adipose tissue –increased lipolysis Muscle –increased protein catabolism in skeletal muscle Kidney -increased erythropoietin synthesis Respiration- increased central stimulation of respiration Energy metabolism -increased BMR,increased oxygen consumption,increased heat production stimulation of Na-K-ATP ase

Wolff-chaikoff effect Iodine deficiency increases thyroid blood flow & upregulates the NIS , stimulating more efficient uptake. Excess iodide transiently inhibits thyroid iodide organification ,a phenomenon known as the wolff- chaikoff effect

The functional unit of thyroid is thyroid follicle. Normal follicle

Thyroid follicle with high TSH stimulation Thyroid follicle with out TSH

High T3 or T4 gives decreased TSH subunit synthesis inactive thyrotrophs may lose the capacity to respond to reduced T3 or T4 levels

derives from the median eminence of the hypothalamus somatostatin TRH inhibits TSH release potentiates the effect of thyroid hormones on thyrotrophs, ie thyroid hormone has inhibitory effects on TSH release derives from the median eminence of the hypothalamus thyrotropin releasing hormone, ie stimulates TSH release

Primary hypo thyroidism Secondary hypotyroidism Iodine deficiency Hasimoto’s thyroiditis Thyroidectomy Radiation therapy Drugs-lithium,antithyroid drugs and PAS Absent or ectopic thyroid gland Dyshormonogenesis TSH receptor mutation Hypopituitarism Tumors,pituitary surgery, irradiation/infiltration, sheehan’s syndrome & isolated TSH deficiency Hypothalamic disease Trauma & infiltration

cretinism congential absence of T3 and T4 or chronic iodine deficiency during childhood retarded growth sluggish movements mental deficiencies

Interesting, right? This is just a sneak preview of the full presentation. We hope you like it! To see the rest of it, just click here to view it in full on PowerShow.com. Then, if you’d like, you can also log in to PowerShow.com to download the entire presentation for free.