8534 Comorbid Autoimmune Thyroid Disease in a Parent-Child Pair with Resistance to Thyroid Hormone Beta

Abstract Disclosure: A.H. Smith: None. E. Grethen: None. Resistance to Thyroid Hormone Beta (RTHβ) is an inherited disorder caused by mutations in the gene encoding the thyroid hormone receptor beta protein, resulting in decreased binding affinity to triiodothyronine in certain tissues. An asymptomatic individual with RTHβ may demonstrate abnormalities in serologic evaluation of thyroid hormone markers. When other thyroid pathologies present with RTHβ, however, the diagnosis and management can quickly become complex. Below we discuss two cases, a parent-child pair, with different presentations of presumably the same RTHβ gene. Patient 1 is a 61-year-old male with a history of autoimmune thyroiditis several years ago, presenting for continuation of care; he has had levothyroxine doses gradually uptitrated to over 3 mcg/kg to normalize TSH despite persistently high free T4 levels. He also has a family history notable for six out of seven siblings with autoimmune thyroid disease. Patient 2 is a 44-year-old male who was evaluated for hypothyroidism and difficulty with levothyroxine titration. Prior to consultation, he underwent radioactive iodine ablation for presumed Grave’s disease, followed by total thyroidectomy due to persistent symptoms. Post-operatively, he required high doses of levothyroxine to maintain normal TSH levels; however T4 and T3 levels remained high. Both patients confirmed proper levothyroxine supplementation. Further analysis of both patients demonstrated mildly elevated TBG and normal alpha subunit. Patient 2 pursued genetic testing while patient 1 declined, revealing a heterozygous R320S mutation in the TR-beta protein. Further testing and genetic counseling were offered to other family members. Propranolol and levothyroxine supplementation were titrated accordingly; triiodothyronine administration was attempted but did not alleviate symptoms. These two cases demonstrate the increased risk of autoimmune thyroid disorders in patients with RTHβ and the unique challenges they pose in accurate diagnosis. Autoimmune thyroiditis in RTHβ, with goiter, elevated T4 and elevated T3 levels, can be easily confused with Grave’s disease, especially when TSH is suppressed during active thyroiditis. Timely diagnosis can aid in targeted management of sequelae, prevent ineffective treatments for comorbid conditions which may mimic other diseases, and provide anticipatory/preventive guidance for family members to help with screening. Presentation: 6/1/2024