Two Mutations in Thyroid Hormone Receptor Beta Gene (P453A and C36Y) in a Family with Resistance to Thyroid Hormone with Comorbid Myotonic Dystrophy.

The co-occurrence of resistance to thyroid hormone beta (RTHβ) and myotonic dystrophy type 1 (DM1) was observed in a Japanese family. Two mutations, P453A and C36Y, were identified in the thyroid hormone receptor beta (THRB) gene. Whereas family members with THRB exhibited RTHβ, two members with THRB but without THRB had normal thyroid function. Two members, one with RTHβ and the other without, had a triplet expansion in the dystrophia myotonia protein kinase gene, a hallmark of DM1. The member with both RTHβ and DM1 developed atrial fibrillation at the age of 16 years, suggesting a synergistic impact on the heart.