Resistance to Thyroid Hormone beta (RTHβ) due to homozygous THRB defects is exceptionally rare, with only five cases reported worldwide; cardiac dysfunction, which can be life-threatening, is recognised in the disorder. Here we describe the clinical, metabolic, ophthalmic and cardiac findings in a nine-year old boy harbouring a biallelic THRB mutation (R243Q), along with biochemical, physiological and cardiac responses to carbimazole and triiodothyroacetic acid (TRIAC) therapy. The patient exhibits recognised features (goitre, non-suppressed TSH levels, upper respiratory tract infections, hyperactivity, low body mass index) of heterozygous RTHβ, with additional characteristics (dysmorphic facies, winging of scapulae) and more markedly elevated thyroid hormone levels, associated with the homozygous form of the disorder. Notably, an older sibling with similar clinical features and probable homozygous RTHβ, had died of cardiac failure at age 13 yrs. Features of early dilated cardiomyopathy in our patient prompted combination treatment with carbimazole and TRIAC. Careful titration of therapy limited elevation in TSH levels and associated increase in thyroid volume. Subsequently, sustained reduction in thyroid hormones with normal TSH levels was reflected in lower basal metabolic rate, gain of lean body mass and improved growth and cardiac function. A combination of anti-thyroid drug and TRIAC therapy may prevent hyrotoxic cardiomyopathy and its decompensation in homozygous or even heterozygous RTHβ in which life-threatening hyperthyroid features predominate.