Resistance to Thyroid Hormone Due to a Missense Heterozygous Mutation in the

Background: Resistance to thyroid hormone β is a rare condition that causes decreased sensitivity to thyroid hormones in target tissues. We present the case of a young female diagnosed with a heterozygous missense mutation in the THRB gene, focusing on diagnosis, management, and preventing potential complications. Case Report: A 20-year-old female presented for evaluation of abnormal thyroid function tests showing elevated free thyroxine and non-suppressed thyroid-stimulating hormone for 5 years. On examination, there was no evidence of exophthalmos, thyromegaly, neck mass, or lymphadenopathy. Repeat lab work at the time of her visit showed a thyroid stimulating hormone of 1.720 uIu/mL (reference range: 0.450-4.500 uIu/mL), a free thyroxine of: 1.98 ng/dl (reference range: 0.8-1.7 uIu/mL), and a free triiodothyronine of: 3.7 pg/mL (reference range: 1.81-4.06 pg/mL). Thyroid autoantibodies, including thyroid-stimulating immunoglobulin and thyroid peroxidase antibodies, were negative. Thyroid uptake and scan showed diffuse, increased uptake. Pituitary MRI did not show evidence of pituitary adenoma. Genetic testing revealed a heterozygous missense mutation in the THRB gene (c.1357 C > G) which is predicted to result in the amino acid substitution p.Pro453Ala. Discussion: In most cases of resistance to thyroid hormone β, increased secretion of thyroid hormones partially compensates for the thyroid hormone resistance, as seen in our patient. In those instances, conservative management is appropriate. Conclusion: It is clinically relevant to identify this condition, given that delayed diagnosis can lead to unnecessary treatments. Given its phenotypic variability, diagnosis is difficult, and management should be focused on clinical presentation.