15- 18 THYROID HORMONES T3 AND T4

Thyroid Hormones, T3 and T4 8

- Chemical class: Amine - Travels in the blood bound to carrier proteins - Secreted continuously but not required for survival - Regulation of secretion - Thyroid Anatomy - Synthesis and storage of T3 and T4 - Target cells and effects of T3 and T4 - Hyperthyroidism and Hypothyroidism

Thyroid Hormones, T3 and T4: Regulation of secretion Release ? Feedback inhibition ?

- Hormonal feedback regulation through hypothalamus/ant. pituitary - Release through secretion of thyrotropin releasing hormone (TRH) from hypothalamus - Feedback inhibition of TRH through blood levels of T3 and T4

Thyroid Hormones, T3 and T4: Thyroid anatomy

- wall of each follicle consists of follicle cells - the hollow interior is filled with glycoprotein fluid, called colloid

Hypothyroidism results in: decreased cellular metabolic rate resulting in: 4

i. decreased body temp. ii. cold intolerance iii. weight gain, but not increased appetite iv. decreased protein synthesis: dry skin, brittle hair and nails

Hyperthyroidism results in: increased cellular metabolic rate resulting in: 5

i. elevated body temp ii. heat and cold intolerance iii. increased protein and fat catabolism (loss of muscle mass and fat reserves) iv. Weight loss + loss of appetite v. High rate of protein synthesis leads to poor quality proteins: dry brittle hair, nails, skin

Increase the rate of associative (aerobic) respiration in cells by:

increasing synthesis and activity of metabolic enzymes

Exerts permissive effects on other hormones bc it

serves to regulate the rate of use of specific biomolecules i metabolic pathways

Steps in synthesis of T3 and T4 4

1. Follicle cell absorbs dietary iodine from the blood ad secretes it to the colloid 2. Follicle cells manufacturing secrete thyroglobulin to the colloid 3. Enzymes within the colloid combine iodine and thyroglobulin to form inactive T3 and T4 4. Inactive T3 and T4 are stored in the colloid

Thyroid Hormones, T3 and T4: Synthesis of T3 and T4 4

1. Synthesis and storage of T3 and T4 occurs within the follicles 2. Follicles can store 3-4 months supply of T3 and T4 3. Steps in synthesis of T3 and T4 4. Secretion of T3 and T4 from thyroid follicles

Secretion of T3 and T4 from thyroid follicles 5

1. T3 and T4 are transported into follicle cells 2. Enzymes in the follicle cells remove the globulin portion of the inactive T3 and T4 rendering it active and lipid soluble 3. T3 and T4 is transported out of follicle cells to the blood, travel to target cells 4. T3 and T4 diffuse into target cells 5. Within target cells, less active T4 is converted to more active T3 based on the needs of the cell

Thyroid Hormones, T3 and T4: Target cells and Effects of T3 and T4 3

1. Target cels include most cells of the body 2. Increase the rate of associative (aerobic) respiration in cells increasing oxygen consumption 3. Exerts permissive effects on other hormones

Hypothyroidism results in: 5

1. decreased cellular metabolic rate 2. fatigue, slowed reflexes, due to decreased neural activity 3. slow heart rate 4. localized edema 5. Glycoprotein deposits under skin produces myxedema under eyes and chin

Thyroid Hormones, T3 and T4: Secreted continuously but not required for survival

1. essential for normal growth and development 2. Has permissive interactions with several other hormones (GH, estrogen, insulin)

Hyperthyroidism results in: 4

1. increased cellular metabolic rate 2. Increased neural excitability 3. Glycoprotein deposits behind eyeballs produces exophthalmus