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Phys Spring 4 Lec 6
Thyroid Metabolic Hormones
Question | Answer |
---|---|
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 |