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Phys Spring 4 Lec 6

Thyroid Metabolic Hormones

What hormones do the thyroid produce? T3 (triiodothyronine), T4 (thyroxine), and calcitonin
Which cells secrete calcitonin? What is the fxn of calcitonin? C-cells; Involved in Ca regulation
What is the effect of T3 & T4 on metabolic rate? Inc metabolic rate
What stimulates growth of the thyroid gland and T3 & T4 production? TSH from anterior pituitary
Where is the thyroid gland located? Directly beneath the larynx, in front of and on both sides of the trachea
Describe the structure of the thyroid gland follicles The thyroid gland is composed of follicles that are lined w/a single layer of epithelial cells and have a center filled w/a protein colloid. B/n the follicles are capillaries and C-cells
Discuss how thyroid hormones are produced Thyroid ECs produce thyroglobulin (glycoprotein) --> thyroglobulin is secreted into center of follicule and the tyrosines are iodinated --> Iodinated thyroglobulin is brought back into the cell and cut by proteases --> T4 & T3 diffuse into blood
How does iodide (I-) get into the cell? Na+ & I- are co-transported into the cell
What are the 4 actions of TSH? 1. Stimulates the Na+/I- symporter (iodide pump); 2. Stimulates pinocytosis & proteolysis of iodinated thyroglobulin; 3. Stimulates T3 & T4 secretion; 4. Stimulates growth of ECs of the follicle
What is the % of T3 vs. T4 that is released by the thyroid into the blood? 90% T4, 10% T3
What is the first thing that happens when T3 & T4 diffuse into the blood? They are immediately bound to plasma proteins, mostly thyroxine-binding globulin (TBG)
Discuss why measuring total plasma T4/T3 is not a good index of thyroid hormones Only about 0.02% of total T4 is free in the plasma, the rest is bound to TBG
What are the standard measurements for thyroid fxn? Free T4 & TSH levels
Discuss uptake of T4/T3 by tissue cells T3/4 are slowly released from plasma proteins --> diffuses into tissue cells --> Almost all T4 is then deiodinated to T3
What is the half-life of plasma T4 8 days
How does T3/4 affect transcription? T3/T4 binds to a receptor in/on DNA strand --> gene transcription is then inc or dec
What happens when you have prolonged injection of thyroxine (T4)? Causes an initial inc in basal metabolic rate, peaking ~day 10. After day 10, basal metabolic rate slowly returns to original levels --> this is d/t delays between plasma binding T4, delivery to target cell, and gene activation, transcription, and effects
Discuss how thyroid hormones effect Na/K/ATPase in regards to basal metabolic rates Plasma membrane Na/K/ATPase activity inc, and the plasma membrane leaks more Na & K
Discuss how thyroid hormones effect B-adrenergic receptors in regards to basal metabolic rates B-adrenergic receptors inc --> inc HR, tremor, anxiety, agitation
Discuss the general effect of thyroid hormones on basal metabolic rates Can inc basal metabolic rate up to 60-100% d/t the inc production of heat
Discuss how thyroid hormones effect heat production in regards to basal metabolic rates T3 stimulates thermogenin (mostly in brown fat in infants, small amts in adults) --> activates H+ channel in mitochondria, resulting in heat production BUT NO ATP PRODUCTION!
What are the 5 general tissue actions of thyroid hormones? 1. CHO metabolism inc; 2. Fat metabolism inc; 3. Protein breakdown inc; 4. HR and CO inc; 5. Respiratory rate & GI motility inc
What effect do thyroid hormones have on CHO metabolism? Inc: GI absorption of glucose, cellular uptake of glucose, glycolysis
What effect do thyroid hormones have on fat metabolism? Inc: lipolysis, plasma FFAs & cellular oxidation of FFAs, secretion of cholesterol into bile
What effect do thyroid hormones have on protein metabolism? Protein breakdown inc --> urea nitrogen inc, muscle mass dec
What effect do thyroid hormones have on HR & CO? Metabolic byproducts inc tissue vasodilation, causing inc in HR & CO
Is the effect of thyroxine on the pituitary and hypothalamus (+) or (-) feedback? Negative feedback
Describe Iodine Deficiency Goiter: Iodine deficiency in the diet causes low production of T4/T3, so there is a lack of feedback inhibition on TSH secretion. Excess TSH causes gland enlargement --> goiter
Discuss Graves' Dz A HYPERthyroid autoimmune dz: ABs form to the TSH receptor --> ABs stimulate receptor --> stimulus causes enlargement (goiter) --> goiter produces excess T4/3 --> inhibits natural TSH release
What is the key physical manifestation of Graves' Dz? What causes it? Exophthalmos (protruding eyeballs) - d/t AB rxn w/TSH receptors on the extraoccular muscles/connective tissue
Discuss Hashimoto's Thyroiditis A HYPOthyroid autoimmune dz: ABs against thyroglobulin, thyroid peroxidase, or TSH receptors --> ABs gradually destroy follicles --> dec T4/T3 --> inc TSH --> goiter can form from remaining undamaged tissue
What is the most common form of hypothyroidism in North America? Hashimoto's Thyroiditis
Is Hashimoto's Thyroiditis more common in men or women? Women
What would a lab test show before the sxs of Hashimoto's Thyroiditis occur? Elevated TSH
Created by: hclark86



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