Clinical evidence,mechanism, importance and management
Retrospective analysis of 11 patients taking levothyroxine and with thyroid cancer found that 8 patients who had previously undergone a total thyroidectomy developed markedly elevated TSH levels and were clinically hypothyroid after receiving treatment with imatinib. Despite a mean increase in dose of levothyroxine of about 200%, hypothyroidism was reversed in only 3 patients. Thyroid function tests normalised on discontinuing imatinib. Conversely, no effect on thyroid status was seen in 3 patients who had not had their thyroid gland removed (See reference number 1).
The authors postulated that imatinib might increase clearance of thyroid hormones thyroxine and tri-iodothyronine by induction of glucuronosyltransferases (UGTs) (See reference number 1)
The findings from this study appear to be established. TSH levels should be closely monitored in thyroidectomy patients taking levothyroxine if they are given imatinib, anticipate need to increase levothyroxine dose. The authors suggest that in thyroidectomy patients dose of levothyroxine should be doubled before starting imatinib (See reference number 1).
1. de Groot JWB,Zonnenberg BA, Plukker JTM, van Der Graaf WTA, Links TP. Imatinib induces hypothyroidism in patients receiving levothyroxine. Clin Pharmacol Ther (2005) 78, 433–8.