FRI532 Thyroid Hormone Resistance: A Possible New Variant In The THRB Gene

Disclosure: A. Siddiqui: None. Z. Khan: None. T. Ahmed: None. K. Selk: None. Background: Resistance to the Thyroid Hormone (RTH) is caused by reduced target tissue response to thyroid hormone, resulting in the loss of the normal feedback mechanism that suppresses secretion of thyrotropin (TSH). Most cases of RTH are familial, usually inherited in an autosomal dominant manner, with over 100 different pathogenic variants having been described. Clinical Case: A 45-year-old male with history of attention deficit disorder presented for management of hypothyroidism. The patient was Anti-TPO positive, consistent with Hashimoto's thyroiditis and he was treated with varying doses of levothyroxine. Labs on 100 mcg levothyroxine showed a TSH of 7.07 (0.35-4.94 uIU/mL), free t4 1.86 (0.7-1.48 ng/dL), and free t3 4.41 (2.3-4.2 pg/mL). This initially prompted concern for a TSHoma. An alpha subunit level was found to be 0.3 ng/dL and pituitary MRI was within normal limits. Given continued persistent thyroid hormone elevations, further testing for human anti-mouse antibody interference was performed and also unrevealing. The patient continued to report feeling tired and fatigued, with multiple joint pains and inability to focus or concentrate. His levothyroxine was increased to 125 mcg per day. However, even after the adjustment, he still did not notice any change in his symptoms. A trial of addition of liothyronine 5mcg daily was also performed but he self-discontinued this as he did note any improvement. Overall, his findings were highly suspicious for thyroid hormone resistance. Genetic testing for a thyroid hormone resistance beta (THRB) mutation was performed. Direct sequence analysis of the coding region and splice-sites of the THRB gene identified one copy of the c.1368G>T (p.Leu456Phe) variant of uncertain significance. Analysis of this variant using bioinformatics tools for the prediction of the effect of amino acid changes on protein structure and function yielded predictions that this variant is damaging. However, the THRB c.1368G>T (p.Leu456Phe) variant has not been described in online databases or published literature. It also has not been reported in large, multi-ethnic general populations. Conclusion: This is the first case to our knowledge demonstrating the THRB c.1368G>T (p.Leu456Phe) variant gene as likely giving rise to RTH. Further testing of family members has been recommended to help clarify the clinical significance of this finding. Presentation: Friday, June 16, 2023