Puzzling thyroid function test

A 13-½-year-old boy was referred to the Department of Endocrinology as a case of thyrotoxicosis for initiation of antithyroid medication. His chief complaint was a swelling in front of the neck, which was incidentally noted by his mother 2 weeks prior to presentation. He denied any history of symptoms suggestive of hyperthyroidism or ophthalmological involvement. His physical examination was unremarkable except for a grade 2 goitre. Thyroid function test revealed elevated free triiodothyronine and tetraiodothyronine in the face of an unsuppressed thyroid-stimulating hormone level. Technetium-99 uptake scan showed increased uptake indicating enhanced thyroid activity. However, he was clinically euthyroid. This raised the possibility of resistance to thyroid hormones, which was confirmed by documenting similar thyroid function test abnormalities in other members of his family and genetic testing. The family was reassured of the benign nature of the condition.