to the editor: Dr. Kravets provides a good review of hyperthyroidism; however, the article did not discuss what is likely the most common cause of hyperthyroidism: overuse and misuse of thyroid hormone. Many physicians prescribe thyroid hormone therapy for various reasons, such as promoting weight loss in overweight or obese patients, even if not indicated.¹ Thyroid-stimulating hormone or other appropriate thyroid testing should always be performed to verify the need for exogenous thyroid hormone replacement.

in reply: I would like to thank Dr. Scherger for making an important point about the role of exogenous thyroid hormone in causing hyperthyroidism. Iatrogenic hyperthyroidism may be intentional (e.g., patients with thyroid cancer who have clinicopathologic parameters that warrant thyroid-stimulating hormone suppression to prevent cancer recurrence) or inadvertent (e.g., an excessive dose of thyroid hormone prescribed for hypothyroidism).

In patients with no evidence of hypothyroidism, it is inappropriate for physicians to prescribe thyroid hormone therapy to treat obesity or depression. Some patients ingest exogenous thyroid hormone surreptitiously (thyrotoxicosis factitia); others may ingest exogenous thyroid hormone unintentionally when taking contaminated dietary supplements. In addition, there have been cases of accidental ingestion of animal thyroid tissue in hamburgers after the thyroid tissue was inadvertently removed and ground up with neck muscle in a slaughterhouse.¹

When exogenous hyperthyroidism is suspected but cannot be confirmed by patient history, a radioactive iodine uptake of less than 1% and a low serum thyroglobulin level may serve as diagnostic clues. In some patients with thyrotoxicosis factitia, the serum thyroglobulin level may not be suppressed because of a goiter or antithyroglobulin antibodies. In these cases, one can consider measuring fecal thyroxine (T₄) levels. The T₄ levels were shown to be twice the normal value in patients with Graves disease and increased 12- to 24-fold in patients with thyrotoxicosis factitia.²

Unintended iatrogenic hyperthyroidism in patients with hypothyroidism who receive thyroid hormone therapy can be prevented by monitoring thyroid-stimulating hormone levels at least annually and adjusting the dose as needed. Physicians must adhere to the principles of beneficence and nonmaleficence when prescribing thyroid hormone therapy.³ Thyrotoxicosis factitia should be treated by discontinuing exogenous thyroid hormone therapy, providing patient education, and referring for psychiatric evaluation when appropriate.