Exemestane-induced subclinical hypothyroidism : a case report

Several conditions and drugs induce subclinical hypothyroidism. We report the first case of subclinical hypothyroidism in a 65-year-old woman with breast cancer receiving therapy with the third-generation aromatase inhibitor exemestane 25 mg/day for 2 months. The patient presented with complaints of increasing fatigue and weakness since being commenced on exemestane and was taking no other drugs. There was no past history or family history of thyroid disease. Thyroid function tests prior to breast cancer surgery were normal. Detailed clinical examination and laboratory tests to determine the cause of the patient's increasing fatigue and weakness revealed only subclinical hypothyroidism, that is, an elevated level of thyroid-stimulating hormone (thyrotropin, TSH) only. Ultrasonography revealed a normal thyroid gland. Based on a diagnosis of symptomatic subclinical hypothyroidism, the patient was commenced on levothyroxine sodium 50 microg/day and exemestane was withdrawn. Thyroid dysfunction was restored 4 months after her admission with a significant improvement in symptoms. Levothyroxine sodium was withdrawn 6 months later and no recurrence of thyroid dysfunction occurred during a 1-year follow-up. We believe that the increasing fatigue and weakness in our patient might have been associated either with subclinical hypothyroidism or with administration of exemestane (a known adverse effect of the drug) or both. Further studies are required to investigate how exemestane influences thyroid function.