Expert opinion on thyroid complications of new anti-cancer therapies: Tyrosine kinase inhibitors.

Thyroid pathology is the most frequent form of endocrinopathy during tyrosine kinase inhibitor (TKI) treatment. Dysthyroidism occurs in 10% to 80% of cases, depending on diagnostic criteria. In patients with intact thyroid gland prior to TKI treatment, incidence of dysthyroidism is 30-40%, with subclinical presentation in half of cases. It mainly involves hypothyroidism, preceded in 20-40% of cases by transient thyrotoxicosis that may go overlooked. The pathophysiological mechanism is "vascular" thyroiditis induced by the anti-angiogenic action of TKIs. Between 20% and 60% of patients receiving levothyroxine ahead of TKI treatment show increased levothyroxine requirements. TKIs should not be discontinued because of onset of thyroid dysfunction. Treatment is symptomatic in case of thyrotoxicosis, and levothyroxine replacement therapy is initiated in case of symptomatic hypothyroidism or TSH>10mIU/L. During TKI treatment, TSH should be assayed monthly, or at end of off-period (i.e., day 1 of new cycle after interruption), for the first 6 months, then every 2-3 months or in case of clinical signs of dysthyroidism. In patients already treated for hypothyroidism, TSH should be assayed monthly for 3 months, then every 3 months throughout treatment. At TKI termination, remission of hypothyroidism is possible but unpredictable, and progressive discontinuation of levothyroxine may be considered under monitoring. Teamwork between oncologists and endocrinologists improves screening and treatment of thyroid dysfunction, enabling the patient to be better accompanied during treatment.