Low serum free thyroxine level in a girl with McCune-Albright syndrome

A 17-year-old girl with McCune-Albright syndrome (MAS) was suspected of having central hypothyroidism based on an inappropriately normal thyroid-stimulating hormone (TSH) and low free thyroxine (fT4). She was clinically euthyroid and her pituitary appeared normal on MRI. Treatment of hypothyroidism with levothyroxine resulted in suppression of TSH with a low fT4 and high free triiodothyronine (fT3) concentration and hence iatrogenic hyperthyroidism was diagnosed. After discontinuation of levothyroxine, the TSH and fT3 normalised while fT4 remained low. Increased conversion of thyroxine (T4) to triiodothyronine (T3) can be part of MAS. Therefore, fT3 and fT4 should both be measured when evaluating thyroid function in patients with MAS.