Bexarotene: A Rare Cause of Misleading Thyroid Function Tests

Bexarotene is a very rare cause of central hypothyroidism (CH) and its effects have been reported to be dose-dependent; however, the available data in the literature on dose-dependent effects are variable. The standard practice of monitoring thyroid function using thyroid-stimulating hormone (TSH) to adjust levothyroxine (LT4) dose does not apply to bexarotene since it causes CH. In CH, TSH is not reliable. Hence free tetraiodothyronine (fT₄) level is used to monitor and adjust the LT4 dose. We report a case of an 81-year-old Caucasian male with cutaneous T-cell lymphoma (CTCL) who was treated with bexarotene. His pre-treatment TSH was normal at 1.6 µIU/mL (reference range: 0.46-4.68 µIU/mL). Post-bexarotene, the total tetraiodothyronine (T₄) level was within the reference range, but a downward trend was noted. Eventually, total triiodothyronine (T₃) dropped to a low level of 0.61 ng/mL (reference range: 0.97-1.69 ng/mL), and LT4 was initiated. Bexarotene dose was increased, but LT4 was not increased by the primary physician who relied on TSH level, which was low, and hence the existing LT4 dose was maintained. The patient had persistent symptoms of hypothyroidism and, eventually, a diagnosis of CH was made. The symptoms of hypothyroidism improved after normalizing fT₄, with an increase in the LT4 dose. This case represents an example of missed CH due to bexarotene, -which led to suboptimal LT4 replacement impacting the quality of life for the patient.