Coexistence of Thyroid Hormone Resistance and Autoimmune Thyroid Disease: Not a Mere Coincidence

Introduction: The coexistence of thyroid hormone resistance and autoimmune thyroiditis was initially thought to be a chance event. In a large cohort study, Barcoff et al. demonstrated an increased likelihood of thyroid autoantibodies in patients with thyroid hormone resistance (RTH). We report a unique case to epitomize the coexistence of these two conditions and discuss the postulated mechanisms. Clinical Case: A 22-year-old woman with a history of Hashimoto’s thyroiditis, attention deficit hyperactivity disorder, and migraine presented to the endocrinology clinic with symptoms of weight loss, fatigue, decreased appetite, and heat intolerance for 4 months. She was diagnosed with Hashimoto’s thyroiditis at age 12 with elevated TSH only and had been on levothyroxine 25 mcg since diagnosis. Physical exam demonstrated a body mass index of 34.14 kg/m2, blood pressure of 138/91 mmHg, pulse of 77 bpm, and an enlarged palpable thyroid gland. Laboratory investigations revealed elevated thyroid peroxidase (TPO) antibody at 234 IU/mL (<9) and thyroglobulin Antibody at 3 IU/mL (<1) with elevated free T4 of 3.76 ng/dL (0.61-1.36), elevated total T4 of 21.58 mcg/dL (6.09-12.23), increased T3 uptake of 51.2% uptake (32-48.4), and upper normal TSH of 3.99 uIU/mL (0.44-5.33). Elevated free T4 was confirmed by the equilibrium dialysis method at 6.2 ng/dL (0.9-2.2) with upper normal TSH at 3.77 uIU/mL. Thyroid ultrasound demonstrated thyromegaly and hypervascularity of the gland. TSH secreting pituitary adenoma was ruled out with a normal alpha subunit of 0.2 ng/ml (0.1-0.6) and molar ratio of the alpha subunit to TSH ratio <1 along with a normal pituitary MRI. She was not taking other medications or supplements. She was diagnosed with thyroid hormone resistance and is undergoing genetic testing to differentiate the THRB genetic mutations from Non-TR-RTH. Her symptoms improved after discontinuing her levothyroxine. Clinical Lesson: Our case highlights the importance of evaluating thyroid disorders properly before starting treatment and illustrates the coexistence of autoimmune thyroiditis with thyroid hormone resistance. Barcoff et al. demonstrated that there was an increased likelihood of thyroid autoantibodies with odds ratio = 2.36 (p = 0.002) in a large patient cohort with RTH, compared to their unaffected first-degree relatives. However, since there was no correlation in increased antibody with increased age and the duration of the disease, the proposed hypothesis did not substantiate chronic TSH stimulation inducing autoimmune response. The proposed mechanism is that the elevated TSH in RTH stimulates the immune system at the TRα level which was demonstrated in murine models with increased activation of thymic function in correlation with TSH level. More research is needed to understand the underlying mechanism of their coexistence.