A novel tyrosine-kinase selective inhibitor, sunitinib, induces transient hypothyroidism by blocking iodine uptake

Sunitinib (sunitinib malate; SU11248; Sutent; Pfizer Inc., New York, NY) is a multitarget inhibitor of tyrosine kinases for the treatment of some human cancers. A myxedematous coma in a patient treated with sunitinib for a gastrointestinal stromal tumor was unexpectedly observed.

Our objective was to evaluate the effect of sunitinib on thyroid function in 24 patients with gastrointestinal stromal tumors.

This was a prospective, observational cohort study.

The study was performed at two tertiary care hospitals.

A total of 24 patients receiving the following cycles of therapy were included in the study: 4-wk daily treatment at the dose of 50 mg orally (ON) and 2-wk withdrawal (OFF).

Thyroid function tests, ultrasonography, and iodine-123 ((123)I) thyroidal uptake were performed at the end of several ON and OFF periods.

After one to six cycles of treatment, 46% of patients developed hypothyroidism. Initially, TSH levels were elevated at the end of ON periods and normalized at the end of OFF periods, but a worsening in following cycles was always observed. Neither echographic alterations nor variations in thyroglobulin and antithyroid autoantibodies were found during the ON and OFF periods. On the contrary, (123)I uptake was significantly reduced at the end of ON periods, with partial or total normalization at the end of OFF periods.

A high prevalence of hypothyroidism, very severe in some cases, was observed during sunitinib. Significant variations in (123)I uptake strongly suggest that the underlying mechanism is an impaired iodine uptake. The absence of thyroid autoimmunity, the lack of a preceding transient hyperthyroidism, and the normal echographic pattern exclude autoimmune and/or destructive mechanisms. Patients on sunitinib should be strictly monitored for the appearance of hypothyroidism and promptly treated.