Review Question - QID 204427

This patient has signs and symptoms of hyperthyroidism. Low uptake of radioactive iodine in conjunction with a low TSH and thyroglobulin are characteristic of excessive doses of levothyroxine (or factitious thyrotoxicosis).

Patients with factitious thyrotoxicosis have signs and symptoms of thyrotoxicosis without goiter or exophthalmos. Consider this diagnosis in a patient who has access to levothyroxine such as a nurse or a pharmacist. Recall the other common causes of hyperthyroidism: Graves' disease (diffuse toxic goiter), Plummer's disease (multinodular toxic goiter), toxic thyroid adenoma, Hashimoto's thyroiditis (which may cause transient hyperthyroidism before causing hypothyroidism), postpartum thyroiditis, and iodine induced hyperthyroidism. All these patients may present with the classic clinical features including nervousness, hand tremor, sweating, weight loss, diarrhea, and palpitations. Treatment may be pharmacologic via thionamides such as PTU and methimazole, or may consist of radioactive iodine 131, and even surgical subtotal thyroidectomy.

Ross reviews syndromes of thyrotoxicosis with low radioactive iodine uptake. Ectopic hyperthyroidism includes factitious thyroid hormone ingestion, struma ovarii, and, rarely, large deposits of functioning thyroid cancer metastases.

Reid et al. discuss the diagnosis and treatment of hyperthyroidism. The most common cause of hyperthyroidism is Graves’ disease. Other common causes include thyroiditis, toxic multinodular goiter, toxic adenomas, and side effects of medications. The diagnostic workup begins with a thyroid-stimulating hormone level test. When test results are uncertain, measuring radionuclide uptake helps distinguish among possible causes.

Illustration A depicts the endocrine pathways of the thyroid gland. The hypothalamus secretes TRH which causes the pituitary to release TSH. TSH then acts on the thyroid to release T3 and T4 in the blood.

Incorrect Answers:
Answer 1: In Graves' disease, the patient would have a goiter and the radioiodide scan would show diffuse uptake.
Answer 2: In toxic multinodular goiter (Plummer's disease), there would be patchy uptake on the thyroid scan. This form of hyperthyroidism is also more common in elderly patients.
Answer 4: Thyroid carcinomas are usually non-functioning nodules.
Answer 5: This patient's symptoms are indicative of hyperthyroidism, not hypothyroidism. Iatrogenic hypothyroidism occurs after radioiodine therapy, thyroidectomy, or may occur with certain medications such as Lithium.