Resistance to Thyroid Hormone (RTH) is a condition caused by tissue hyposensitivity to the effects of circulating thyroid hormone, and may be misdiagnosed as hyperthyroidism.We report the first case of RTH in an Irish patient highlighting the clinical features and the pathophysiological mechanism underlying the characteristic laboratory abnormalities found in the condition.We describe an isolated case of RTH initially misdiagnosed as hyperthyroidism, and detail the investigations which ultimately led to the correct diagnosis. Genetic screening of the thyroid hormone receptor beta gene was performed.Thyroid function tests including T3 suppression test and TRH-stimulation test suggested a diagnosis of RTH. Genetic testing failed to demonstrate a mutation in the thyroid hormone receptor.RTH is a rare inherited condition that may be misdiagnosed as hyperthyroidism. The case we describe most likely results from a de novo mutation in an as yet undiscovered gene. RTH should be considered in patients with elevated thyroid hormone levels and normal TSH so that unnecessary and potentially harmful treatment can be avoided.