Thyroid hormones

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

2. Thyroid gland produces two principal hormones … thyroxine & tri iodothyronine 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

10. 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.

11. 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

12. Mechanism of thyroid hormone receptor action

13. 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&glycogenolysis • Adiposetissue –increased lipolysis • Muscle –increased protein catabolism in skeletal muscle • Kidney -increased erythropoietin synthesis • Respiration- increased central stimulation of respiration • Energymetabolism -increased BMR,increased oxygen consumption,increased heat production stimulation of Na-K-ATP ase

14. 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

15. The functional unit of thyroid is thyroid follicle. Normal follicle

16. Thyroid follicle with out TSH Thyroid follicle with high TSH stimulation

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

19. 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

20. Primary hypo thyroidism Secondary hypotyroidism • Iodine deficiency • Hasimoto’sthyroiditis • 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

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

22. 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.