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23 Cards in this Set
- Front
- Back
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What is the pyramidal lobe in the thyroid?
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The two lobes of the thyroid are connected by a bridge of tissue, the thyroid isthmus, and there is sometimes a pyramidal lobe, arising from the isthmus in front of the larynx.
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The thyroid has one of the highest rates of blood flow per gram of tissue of any organ in the body. TRUE/FALSE
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TRUE.
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Regarding iodine metabolism, what is the fate of iodine ingestion?
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It is converted to iodide and absorbed. The minimum daily iodine intake that will maintain normal thyroid function is 150ug in adults, but the average intake in the US is app 500ug/d.
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What organs pricipally take up iodide/
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The thyroid, which uses it to make thyroid hormone, and the kidney, which excrete it in the urine.
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What is the Na+/I- symporter?
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The thyroid cell membranes facing the capillaries contain a symporter, or iodide pump, that transports sodium and iodide. This mechanism is capable of producing intracellular iodide concentrations that are 20-40 times as great as the concentration in plasma.
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Do any other tissues in the body have or utilize a symporter system?
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The salivary glands, the gastric mucosa, the placenta, the ciliary body of the eye, the choroid plexus, and the mammary glands also have one. They transport iodide against a concentration gradient, but their uptake is NOT effected by TSH.
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What happens to iodide in the thyroid gland/
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Iodide is oxidized to iodine and bound to the carbon 3 position of tyrosine residues that are part of the thyroglobulin molecule in the colloid.
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Substantively, what is thyroglobulin/
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It is a glycoprotein made up of two subunits and has a molecular weight of 660,000d.
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What is thyroid peroxidase?
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Thyroglobulin is synthesized in the thyroid cells and secreted into the colloid by exocytosis of granules that also contain thyroid peroxidase, the enzyme that catalyzes the oxidation of iodide and its binding. The thyroid hormones remain part of the thyroglobulin molecules until secreted, colloid is ingested by the thyroid cells, the peptide bonds are hydrolyzed, and free T4 and T3 are discharged into the capillaries.
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In the process of thyroid hormone, what is the first product synthesized?
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The first product is monoiodotyrosine(MIT). MIT is next iodinated in the carbon 5 position to form diiodotyrosine(DIT). Two DIT molecules then undergo an oxidative condensation to form T4 with the elimination of the alanine side chain from the molecule that forms the outer ring.
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What is the normal T4 level in an adult?
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The normal total plasma T4 level in adults is app 8ug/dl(103nmol/L) and the plasma T3 level is app 0.15ug/dl(2.3nmol/L) Large amounts of both are bound to plasma protein(albumin) It is the free hormone that is physiologically active and that inhibit pituitary secretion of TSH.
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Are any other plasma proteins involved in thyroid binding?
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Another protein is transthyretin, as well as a globulin called thyroxine binding globulin(TBG). of the three, albumin has the largest capacity to bind T4, and TBG has the smallest capacity. Transthyretin half life is 2 days, and that of TBG is 5 days.
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Normally, 99.98 % of thr T4 in plasma is bound, the free T4 level is only about 2ng/dl. there is very little T4 in the urine. T3 is not bound to quite as great an extent. TRUE/FALSE
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TRUE.
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What is the significance of a FT4I(free thyroxin index?
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Until recently, it was difficult to measure free T4 and free T3 in the blood directly, so indexes were used for both of them.
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How are T4 and T3 metabolised?
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T4 and T3 are deiodinated in the liver, kidneys and many other tissues. One third of the circulating T4 is normally converted to T3 in adults and 45% is converted to RT3. Only about 13% of the circulating T3 is secreted by the thyroid and 87% is formed by deiodination of T4.
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What is the relationship between thyroid hormone and cholesterol?
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Thyroid hormones lower circulating cholesterol levels. The decrease in plasma cholesterol concentration is due to increased formation of LDL receptors in the liver, resulting in increased hepatic removal of cholesterol from the circulation.
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How do nuclear accidents influence thyroid tissue?
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radioactive isotopes of iodine are major products of nuclear fission, and if fission products are released into the atmosphere as a result of an accident at a nuclear plant, the isotopes spread for considerable distances in the atmosphere because they are more volatile than the other products. large amounts of radioactive iodine destroy thyroid tissue because the radiation kills the cells.
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Is there any treatment given in fallout areas?
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Potassium iodide is regularly instututed to enlarge the iodide pool and depress thyroid uptake to low levels.
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How do antithyroid drugs work?
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They act either by interfering with the iodide trapping mechanism or by blocking the organic binding of iodine. In either case TSH secretion is stimulated by the decline in circulating thyroid hormones, anf goiter is produced. A number of monovalent anions compete with iodide for transport into the thyroid via the Na+/I- symporter.
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What are some anions employed?
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Anions include chlorate, pertechnetate, periodate, biiodate, nitrate and perchlorate.
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What are thiourylenes?
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They are a group of compounds related to thiourea, inhibit the iodination of monoiodotyrosine and block the coupling reaction. Two examples are propylthiouracil and methimazole. Iodination of tyrosine is inhibited because these two substances compete with tyrosine residues for iodine and become iodinated.
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Can iodide itself affect thyroid function?
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Too little or too much iodide cause abnormal thyroid function. in normal individuals large doses of iodides act directly on the thyroid to produce a mild and transient inhibition of organic binding of iodide and hormone synthesis. This effect is called Wolff-Chaikoff effect. As well, too much iodide can reduce the cAMP response to this hormone, and it inhibits proteolysis of thyroglobulin.`
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Are there naturally occurring goitrogens/
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Thiocynates are sometimes ingested with food, and relatively large amounts of naturally occurring goitrogens are present in some foods. Rutabegas, cabbage and turnips contain progoitrin and a substance that converts this compound into goitrin, an active antithyroid compound. Goitrin is formed even if the vegetables are cooked. cabbage goiters do occur, especially in vegetarians.
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