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19 Cards in this Set
- Front
- Back
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How is Iodine captured from the blood into the Thyroid gland?
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It is taken up through the NIS (Na/I symporter) through an active process.
It then goes through Pendrin-1 an I/Cl transporter. |
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How are the thyroid hormones synthesized?
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Tyrosine is iodinated to form MIT and DIT by Thyroperoxidase. Thyroperoxidase then couples MIT and DIT to form Thyroxine(T4) and Triiodothyronine (T3)
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What is T3 composed of? T4?
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Triiodothyronine = MIT + DIT
Thyroxine = DIT + DIT |
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Thyroglobin is endocytosed into the follicle cell from the follicle lumen where it fuses with lysosome that cleaves it into T3 and T4. Once released into the plasma, these hormones are carried by this protein.
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Thyroxine binding globulin
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They thyroid stores large quantities of pro-hormone in the form of TG. There is a reserve stock.
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There is much more T3 than T4 circulating in the blood at any time.
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What are the plasma half-lives of the two Thyroid hormones? What has the greater protein binding affinity?
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T4 = 6-7 days - stronger affinity, longer half-life, but lower bioavailability
T3 = 1 day |
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Describe the conversion of T4 to T3.
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T4 (Thyroxine) is cleaved either at its inner ring or outer ring by 5'-iodinases. Cleaved of the inner ring yields the inactive rT3. Cleavage of the outer ring yields T3.
This occurs mostly in the liver. Watch in patients with liver disease. |
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Describe the 3 types of deiodinase.
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Type 1 converts T4 to T3 in the liver and kidney.
Type 2 occurs intracellularly and converts T4 to T3 in the Brain, Pituitary, and Brown Fat. Type 3 converts T4 to rT3. |
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TSH facilitates every portion of the T4/3 secretion. Why is it used in cancer patients?
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Because it facilitates Iodine uptake. In cancer patients, radioactive iodine is given to kill the cancer.
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What acts to inhibit the Thyrotropic axis?
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Somatostatin inhibits TSH secretion
Dopamine, Glucocorticoids inhibit TSH secretion High Thyroid hormones inhibit TSH and TRH High I inhibits thyroid hormones |
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What is Protirelin?
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A synthetic peptide that stimulates TSH secretion.
Used for diagnostic purposes |
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Describe the regulation of NIS.
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Low Iodide = Higher NIS expression
High Iodide = Lower NIS expression |
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Prolonged decrease in NIS causes a goiter (thyroid hypertrophy). Why?
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Decreased NIS means decreased thyroid hormone production. This causes increased TSH production which has hypertrophic effects on the thyroid.
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What is the consequence of a diet low in iodine?
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Decreased Thyroid hormone production, increased TSH and thyroid hypertrophy.
The opposite is true of diets high in iodine. |
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A person with a normal thyroid can recover quickly from a high iodine diet. What happens if you are predisposed to hypothyroidism or hyperthyroidism? (Wolff-Chaikoff Effect)
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If you are predisposed to hypothyroidism, a high iodine diet with cause decreased NIS and exacerbate the low hormone problem.
In those predisposed to hyperthyroidism, it causes a transient suppression as opposed to a persistent suppression. This route is taken with those with hyperthyroidism. Drugs are given as well. |
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What is the Jod-Basedow phenomenon?
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In contrast to regular thyroid cells, in cancerous nodules, increased iodine exposure increases thyroid hormone levels.
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What is Graves Disease? What causes it?
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Hyperthyroidism - High metabolic rate, sweating, palpitations, tremors
Autoantibodies are produced for the TSH receptor. They bind to and chronically stimulate it causing hyperthyroidism. |
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What is Hoshimoto's Disease?
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Hypothyroidism
There is a destructive auto-antibody against your thyroglobulin which leads to decreased T4/T3 production and increased TSH. This causes hypertrophy and goiter. |
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What drug is important to know that causes hypothyroidism?
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Bexarotene
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