A 34-year-old woman comes to the office with a 2-week history of palpitations, excessive sweating, and anxiety. The patient has obesity and has been unsuccessful in achieving significant weight loss with diet and exercise. In an attempt to lose weight, she has been taking excessive amounts of levothyroxine given to her by a friend. Pulse is 110/min. Physical examination shows sweaty palms and a soft, nontender, normal-sized thyroid gland. Laboratory evaluation reveals suppressed TSH and elevated free thyroxine (T4) . Which of the following additional findings is most likely to be present in this patient?

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11ec160b_3cae_c2d5_9249_777245d87ebf_MD0001_00 This patient has exogenous thyrotoxicosis, which can occur intentionally due to abuse of thyroid preparations (eg, levothyroxine, porcine thyroid extracts) or inadvertently due to errors in treatment of hypothyroidism. Symptoms resemble endogenous hyperthyroidism and include weight loss, tremor, heat intolerance, and tachycardia/palpitations. In addition to elevated serum thyroxine (T4) and suppressed TSH, laboratory testing in exogenous thyrotoxicosis typically shows the following: Radioactive iodine uptake (RAIU): A radioiodine tracer is administered followed by scintigraphy of the thyroid gland; RAIU correlates with the organification of iodine and the synthesis of new thyroid hormone. In exogenous thyrotoxicosis, endogenous thyroid hormone synthesis is suppressed, so RAIU is low. If thyroid ultrasound is performed, Doppler shows decreased blood flow. Thyroglobulin: Thyroglobulin is a large glycoprotein in thyroid follicles that serves as a source of tyrosine residues for thyroid hormone synthesis. Small amounts of thyroglobulin are normally released with thyroid hormone, although greater quantities may be released in states of increased thyroid hormone synthesis or follicular destruction. In exogenous thyrotoxicosis, thyroid metabolic activity is low, and serum thyroglobulin is also low. Triiodothyronine (T3): T3 is derived primarily from T4 by deiodination in peripheral tissues, although small amounts are coreleased from the thyroid with T4. Exogenous levothyroxine is readily converted to T3, so T3 levels are also usually elevated. (Choice A) Elevated RAIU, thyroglobulin, and T3 suggest increased metabolic activity in the thyroid (eg, Graves disease). (Choice B) Elevated thyroglobulin and T3 with a suppressed RAIU suggest the destruction of thyroid follicles and the release of preformed thyroid hormone (eg, subacute thyroiditis). (Choice D) Following thyroidectomy, patients with thyroid cancer are often treated with levothyroxine to achieve a subclinical hyperthyroid state; the suppressed TSH level avoids stimulation of residual malignant thyroid cells. RAIU and thyroglobulin are low because the thyroid is absent, but because the target hyperthyroid state is mild/subclinical, T3 is often normal or only slightly elevated. (Choice E) Thyroid hormone is carried in the circulation bound to plasma proteins, especially thyroid-binding globulin (TBG). TBG is elevated in hyperestrogenic states (eg, pregnancy, estrogen-based contraceptives); thyroid hormone production increases transiently to fill the increased number of binding sites, then activity (eg, RAIU, serum thyroglobulin) returns to normal. Total T3 and T4 are elevated, but free hormone levels are normal. Educational objective: In exogenous thyrotoxicosis, radioactive iodine uptake and serum thyroglobulin level are low, indicating suppression of endogenous thyroid metabolic activity. Serum T3 levels are often elevated.

A 34-Year-Old Woman Comes to the Office with a 2-Week