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Chatper 38: Thyroid & Antithyroid Drugs

During a routine checkup a 24-year-old woman in your clinic reports restlessness and weight loss in spite of increased appetite. She appears to be in good health otherwise. Her lab values show TSH of 0; free T4 (FT4) of 20 mcg/dL (normal 5–13.5 ug/dL); positive for TSH-receptor (thyrotropin-receptor) antibody (TRAb).

In this type of disease, the cause of the hyperthyroidism is the production of an antibody that does which of the following?

(A) Activates the pituitary thyrotropin-releasing hormone (TRH) receptor and stimulates TSH release

(B) Activates the thyroid gland TSH receptor and stimulates thyroid hormone synthesis and release

(C) Activates thyroid hormone receptors in peripheral tissues

(D) Binds to thyroid gland thyroglobulin and accelerates its proteolysis and the release of its supply of T4 and T3

(E) Binds to thyroxine-binding globulin (TBG) and displaces bound T4 and T3

The antibodies produced in Graves disease activate thyroid gland TSH receptors. Their effects mimic those of TSH. The answer is B. Note that TSH is zero in this patient due to the feedback inhibition by high levels of thyroid hormone.

During a routine checkup a 24-year-old woman in your clinic reports restlessness and weight loss in spite of increased appetite. She appears to be in good health otherwise. Her lab values show TSH of 0; free T4 (FT4) of 20 mcg/dL (normal 5–13.5 ug/dL); positive for TSH-receptor (thyrotropin-receptor) antibody (TRAb).

The decision is made to begin treatment with methimazole. Methimazole reduces serum concentration of T3 primarily by which of the following mechanisms?

(A) Accelerating the peripheral metabolism of T3

(B) Inhibiting the proteolysis of thyroid-binding globulin

(C) Inhibiting the secretion of TSH

(D) Inhibiting the uptake of iodide by cells in the thyroid

(E) Preventing the addition of iodine to tyrosine residues on thyroglobulin

The thioamides (methimazole and propylthiouracil) act in thyroid cells to prevent conversion of tyrosine residues in thyroglobulin to MIT or DIT. The answer is E.

During a routine checkup a 24-year-old woman in your clinic reports restlessness and weight loss in spite of increased appetite. She appears to be in good health otherwise. Her lab values show TSH of 0; free T4 (FT4) of 20 mcg/dL (normal 5–13.5 ug/dL); positive for TSH-receptor (thyrotropin-receptor) antibody ...

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