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24 Cards in this Set
- Front
- Back
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What is the structure of thyroid hormone?
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tyrosine and iodide
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What's the daily recommended value for iodide?
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150 mcg, 200 mcg during pregnancy. 75 mcg are taken up by the thyroid, the rest is excreted in the urine.
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How is thyroid hormone made?
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1. iodide trapping in the follicle colloid.
2. TPO (thyroidal peroxidase) changes iodide to iodine (non-ionic form). 3. iodine iodinates the tyrosine residues on the thyroglobulin molecule to form diiodotyrosine (DIT) and monoiodotyrosine (MIT) 4. Two DIT form T4, a DIT + a MIT form T3 5. hormones are released from the thyroid by endocytosis and proteolysis. 5 T4: 1 T3 is spit out. |
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How is thyroid hormone transported in the blood?
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almost completely bound to thyroxine-binding globulin (TBG). Only unbound thyroid hormone is active.
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What is a high TBG state and what can it be caused by?
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High levels of TBG, but the thyroid adjusts so the levels of free (and therefore active) thyroid are normal.
Caused by estrogen (Oral contraceptives, pregnancy), acute/chronic active hepatitis, acute intermittent porphyria, hereditary. |
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What can cause a low TBG state?
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androgens, glucocorticoids, nephrotic syndrome, hereditary
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How is thyroid hormone normally metabolized?
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T4 --> T3 in liver, kidneys, other target cells (5'-deiodinase)
In illness/starvation, rT3 (inactive) can be made to save energy. Some T4 is conjugated, deaminated, or decarboxylated, and then excreted as glucuronide and sulfate. |
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How does thyroid hormone act on cells?
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T3 binds to receptors in the nucleus that control gene expression.
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How is synthetic T4 (L-thyroxine) absorbed?
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in duodenum and ileum
*modified by foods, drugs (iron, calcium preps in particular), and intestinal flora. 80% is absorbed. |
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What can affect the metabolism of synthetic thyroid hormone?
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drugs that induce hepatic microsomal enzymes increase its metabolic clearance (rifampin, phenobarbital, carbamazepine, phenytoin). A normal person can be euthyroid due to compensatory thyroid hormone production.
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What synthetic thyroid formulations are used on a regular basis?
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L-thyroxine (T4, oral or injectable, most common)
Liothyronine (T3, oral or injectible) Can't justify a more expensive mix of T4 and T3 since T4 is just chanced to T3 anyway. |
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What are the types of antithyroid agents we talked about?
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thionamides (methimazome and propylthiouracil)
iodides iodinated contrast media (ipodate and iopanois acid) radioactive iodide (I-131) beta-blockers (symptomatic) |
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What's the difference between Methimazole and propylthiouracil?
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PTU, antithyroid agent.
incomplete absorption/ large first-pass short plasma halflife but accumulates in the thyroid--> give 3x a day. More protein-bound: used in pregnant women because it doesn't cross the placenta as readily. Inhibits deiodination of T4 to T3 in the periphery, while MMI does not. MMI, antithyroid agent. completely absorbed, slower exretion. short plasma halflife but is accumulated in the thyroid gland--> Single daily dose. Most often used. |
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What's the MOA of thionamides?
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inhibit PTO (iodine organification)
block coupling of iodotyrosines PTU: inhibits deiodination of T4 to T3 in the periphery Do not block iodide uptake Slow onset, depletion of T4 stores may require weeks. |
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What are AEs of thionamides?
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3-12% have maculopapular pruritic rash and GI side effects.
rare: vasculitis, arthralgia, lupus-like reaction, cholestatic jaundice, hepatitis, lymphadenopathy. **most dangerous: agranulocytosis (low neutrophils) --> fatal infections. 50 % cross-reaction between MMI and PTU. |
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How are iodides used for thyroid trouble?
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at pharmacological doses, iodides inhibit the release of thyroid hormones (inhibit thyroglobulin proteolysis?).
Rapid improvement (2-7 days). Also decreases vascularity and size of a tumor, thereby preparing it for surgery. |
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What are the AEs of iodides?
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uncommon.
include acneiform rash, swollen salivary glands, conjunctivitis, rhinorrhea, drug fever, metallic taste, bleeding disorders, anaphylactoid reacitions. |
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How are iodinated contrast media used for thyroid trouble?
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ipodate and iopanoic acid inhibit 5'-deiodinase, and so inhibit the conversion of T4 to T3.
also inhibit thyroid hormone release (pharmacological iodide levels). Useful in thyroid storm. Relatively nontoxic. |
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How is radioactive iodide used for thyroid trouble?
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used for thyrotoxicosis, absorbed and concentrated in the thyroid, destroys thyroid parenchyma.
AE: many patients end up hypothyroid. Safe and effective, no genetic damage, leukemia or neoplasia-- preferred option by many endocrinologists. Do not give to pregnant women --> neonatal hypothyroid. |
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How are beta-blockers used for thyroid trouble?
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symptomatically for thyrotoxicosis: excess thyroid increases beta-adrenergic receptors and so increases the sympathetic tone.
propanolol, atenolol. No therapeutic effects on the gland itself. |
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How is surgery used for thyroid trouble?
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Last resort for hyperthyroid.
Rarely used, because if the patient isn't well prepared (ie returned to euthyroid), surgery may precipitate a thyroid storm. |
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Q1: What is the daily adult requirement of iodide?
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150 mcg
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Q2: What's the oral absorption of L-T4?
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80 %
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Q3: What's the mechanism of action of methimazole?
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inhibits thyroid hormone synthesis
(does not inhibit release or T4 conversion or iodide uptake, nor does it destroy the thyroid parenchyma) |
