Notes on Thyroid Hormones

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Thyroid Hormone Needed:

• Thyroid hormones needed for growth and maturation.

• Daily intake is in form of iodide ion.

Thyroid Hormone Secretion:

• Contains follicle cells that create and release thyroid hormone: T3 and T4.

Synthesis of Thyroid Hormones:

Iodide transport:

• Active pump on follicle brings in iodine; blocked by perchlorate and thiocyanate.

Thyroglobulin Synthesis:

• A large protein made on ribosomes, glycosylated in ER, vesicle-packaged in Golgi apparatus.

• Peroxidase catalyzes oxidation (I- to I0).

Iodination:

• Less than 20% of tyrosine residues on thyroglobulin are iodinated; DIT, MIT produced.

Coupling:

• Peroxidase couples iodinated tyrosine in thyroglobulin molecule; DIT+DIT=T4; MIT+DIT=T3; abundant iodine = T4 production; scarce iodine = T3 production.

• Hormone is then stored in follicular colloid to last 2-3 months.

Secretion of Thyroid Hormone:

Endocytosis:

• Follicular colloid taken back into follicle by endocytosis.

Fusion:

• Endocytosed material fuses with lysosomes.

• Proteolysis of thyroglobulin into T3, T4, MIT, DIT, and more.

Secretion:

• T3/T4 secreted into blood. More T4.

Deiodination:

• Iodine removed from DIT and MIT, but not from T3 and T4; iodine recycled back.

Transport of iodine into blood:

• T4 half-life greater than T3 half-life.

• Protein-bound thyroid hormones act as reserves.

T3 more active form; T4 more in circulation.

Actions of Thyroid Hormones:

• Increase BMR.

• Growth and maturation: brain and GH.

• Increased lipid metabolism.

• Increased CHO metabolism: increased glucose absorption from SI.

• Increased number and affinity of beta-adrenergic cardiac receptors.

Control of Thyroid Hormone Secretion:

• Decreased T4 causes TRH to stimulate TSH secretion.

• TSH targets the thyroid gland; increases T4 secretion.

• T4 creates negative feedback; T3 has no effect on TRH.

TSH Effects on Thyroid:

• TSH increases rate of thyroid hormone production, including degradation.

• Causes increased blood flow to and increased hypertrophy of thyroid gland.

• Excessive TSH = goiter.

Hypothyroidism:

• Decreased BMR and O2 consumption, decreased mental capacity, increased plasma cholesterol, edema, hoarse voice, jaundice, constipation, anemia.

Hyperthyroidism:

• Increased BMR and O2 consumption, excitability, irritability, restlessness, increased HR, Grave's disease.

Additional Readings:

Basic Endocrinology

1. Introduction to Endocrinology
2. Hypothalamic-pituitary system
3. Adrenal Hormones
4. Antidiuretic Hormone (ADH) and ECF Regulation
5. Endocrine Pancreas
6. Growth Hormone
7. Adrenal Medulla
8. Hormonal Control of Calcium and Phosphate
9. Thyroid Hormones
10. Hormones of Male Reproduction
11. Hormones of Female Reproduction
12. Fluid Compartments of the Body
13. Notes on Hypothalamus Anterior Pituitary and Thyroid
14. Additional Notes on Female Reproduction
15. Hormonal Signaling Pathways
16. FAQ on Adrenal Hormones
17. FAQ on Male Reproduction
18. Synthesis and Deficiencies of Adrenal Hormones
19. Significance of Glycosylated Hemoblogin (HbA1c)
20. Significance of Measuring Albumin while with Calcium Levels
21. Stepwise Approach to Treatment of Ascites
22. How to differentiate between Diabetes Insipidus vs Psychogenic Polydipsia

Related Topics

1. Histology of the Endocrine System
2. Histology of the Male Reproductive System
3. Histology of the Female Reproductive System

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