Summary

A young man was diagnosed with hyperthyroidism 10 years prior to current presentation after a random health screening revealed an elevated free thyroxine (fT4) of 36.9 pmol/L. During that time, he saw multiple physicians and was treated with carbimazole intermittently. His repeat thyroid function tests showed persistently elevated fT4 ranging 25–35.7 pmol/L and non-suppressed thyroid-stimulating hormone (TSH) concentrations of 6.78–22.1 mIU/L. He had a smooth, firm and non-tender goitre. At our institution, laboratory interference was first excluded by serial dilution study (TSH) and retesting of TSH and fT4 on alternate assay, which gave reproducible results. His normal α-subunit and sex hormone binding globulin, partially suppressed TSH by high dose triiodothyronine (T3), and positive TSH response to thyrotropin-releasing hormone stimulation were consistent with resistance to thyroid hormone syndrome. The diagnosis was confirmed by direct sequencing of thyroid hormone receptor-β gene, revealing a heterozygous R320 L mutation that causes reduced T3 affinity and reduced corepressor dissociation.