Atrophic hypothyroidism associated with thyrotropin receptor antibodies.

Hypothyroidism is most frequently caused by Hashimoto's thyroiditis. While thyrotropin receptor antibodies are well-known in Graves' disease-induced hyperthyroidism, their role in hypothyroidism is emerging. We report the case of a 37-year-old woman with facial and periorbital edema, weight gain, and hoarseness suggestive of hypothyroidism. Elevated thyroid stimulating hormone (TSH; 131mIU/L) and decreased free thyroxine (0.2ng/dL, 2.6pmol/L) levels confirmed this diagnosis. Laboratory findings showed dyslipidemia, impaired renal function, elevated creatine phosphokinase levels, and anemia. Despite negative results for thyroid peroxidase (TPO-Ab) and thyroglobulin (TgAb) antibodies, positive thyrotropin receptor antibodies (11IU/L) suggested an autoimmune etiology. Thyroid ultrasound confirmed atrophic thyroiditis. Levothyroxine treatment improved the symptoms and laboratory values. Two months after diagnosis, the patient became pregnant. Anemia relapsed during pregnancy, thyrotropin receptor antibodies levels normalized, and postpartum follow-up revealed stable thyroid function without affecting the newborn thyroid function. This case underscores the diagnostic challenges of negative TPOAb and TgAb hypothyroidism, which is often misdiagnosed as Hashimoto's thyroiditis, and highlights the need for functional discrimination in thyrotropin receptor antibodies assays. Thyrotropin receptor antibodies measurements can help in the differential diagnosis of rapidly evolving hypothyroidism in women with negative TPO-Ab and thyrotropin receptor antibodies levels, especially in women of childbearing age. Further research is essential to understand thyrotropin-binding inhibitory immunoglobulin-mediated hypothyroidism and to develop targeted therapies.