A 21-year-old woman comes to the office with 5 months of fatigue, excessive sweating, and palpitations. The patient is slightly overweight and had previously been unsuccessful in losing weight with caloric restriction and exercise. However, she has lost weight during the past 6 months with an over-the-counter weight loss remedy. The patient has had no menstrual periods for the last 3 months. She is not sexually active. Her blood pressure is 136/70 mm Hg and pulse is 100/min. Her BMI is 26 kg/m². The patient has lid lag but no proptosis. Examination of the thyroid shows a small gland without nodules or tenderness. Laboratory findings are as follows: Radioiodine uptake by the thyroid gland is diffusely decreased. Which of the following would most likely be found in this patient?

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11ec25c8_d3bd_7b81_bc81_0f9291906acd_MD0003_00 This patient, with fatigue, sweating, palpitations, and weight loss, has features of hyperthyroidism. The first step in evaluating hyperthyroidism is to measure serum TSH and free T4 levels. In the absence of a TSH-secreting pituitary adenoma (secondary hyperthyroidism), patients with hyperthyroidism will have an elevated T4 and suppressed TSH. The most common cause of hyperthyroidism is Graves' disease, which is caused by an autoantibody to the TSH receptor and is characterized by a diffuse goiter and ocular abnormalities (proptosis, periorbital edema). Patients with clear features of Graves' disease may be managed accordingly, but those without require further investigation. Patients with undiagnosed hyperthyroidism may be evaluated further with radioactive iodine uptake (RAIU), usually with scan. A high RAIU suggests de novo hormone synthesis due to Graves' disease (diffusely increased uptake) or toxic nodular disease (nodular uptake). In contrast, a low RAIU suggests either release of preformed thyroid hormone (ie, thyroiditis) or exogenous thyroid hormone intake. In such cases, the serum thyroglobulin level can make the distinction: elevated thyroglobulin is consistent with endogenous thyroid hormone release whereas decreased thyroglobulin suggests exogenous or factitious thyrotoxicosis. In this patient, it is likely that her weight loss supplement contains thyroid hormone derived from porcine or other animal sources. (Choice A) Antithyroid peroxidase antibodies are seen in Hashimoto thyroiditis, which usually presents with a palpable goiter and clinical hypothyroidism (rather than hyperthyroidism). RAIU is not typically part of the evaluation of hypothyroidism but would generally show low uptake in patients with Hashimoto thyroiditis due to autoimmune thyroid destruction. Rarely, patients with Hashimoto thyroiditis can present with a transient hyperthyroid state (due to inflammation) early in the course of the disease. Factitious thyrotoxicosis is much more likely in this patient who developed manifestations of hyperthyroidism shortly after she started taking an over-the-counter weight loss remedy. (Choice B) Subacute thyroiditis (de Quervain's thyroiditis) is characterized by fever, neck pain, thyroid tenderness, and an elevated erythrocyte sedimentation rate. In most cases, hyperthyroid symptoms fade in <8 weeks as the thyroid gland becomes depleted of preformed hormone. (Choice C) TSH is comprised of 2 subunits, an alpha-subunit (common to TSH, FSH, LH, and hCG) and a thyroid-specific beta-subunit. Many TSH-secreting pituitary adenomas overproduce the alpha-subunit, and an elevated ratio of alpha-subunit to TSH suggests a pituitary adenoma. These patients would have an elevated TSH, elevated free T4, and normal or increased RAIU. (Choice D) This patient's absence of ophthalmopathy (eg, proptosis, periorbital edema) and low RAIU make Graves' disease unlikely. Educational objective: In hyperthyroidism, increased radioactive iodine uptake (RAIU) suggests de novo thyroid hormone synthesis, whereas decreased RAIU suggests release of preformed hormone or exogenous hormone intake. Thyrotoxicosis due to exogenous thyroid hormone is characterized by low serum thyroglobulin levels. References: Factitious thyrotoxicosis and herbal dietary supplement for weight reduction. (http://www.ncbi.nlm.nih.gov/pubmed/18363120)

A 21-Year-Old Woman Comes to the Office with 5 Months