Resistance to thyroid hormone in a child with thyroid agenesis: A case report with review of the literature.

Introduction: The coexistence of thyroid dysgenesis and resistance to thyroid hormone (RTH) is a very rare occurrence. The current study aims to report a unique case of thyroid agenesis with RTH in a pediatric patient. Case report: A 5-year-old male patient presented with poor feeding, excessive somnolence, and a noticeable umbilical hernia since the age of 2 months. He was initially diagnosed as a case of congenital hypothyroidism, and since then, he had been placed on thyroid replacement therapy. No further investigations were conducted until the age of 5 years. Recent laboratory findings revealed an elevated TSH level (42.41 µIU/mL). X-ray examination showed delayed bone age (30 months). Ultrasound (US) examination demonstrated the complete absence of thyroid lobes, isthmus, and ectopic thyroid tissue, but small 2.7 x 2.5-mm non-echoic, cystic, and hypo-vascular nodules were seen in the bed of the right thyroid lobe. The patient was kept on thyroid replacement therapy (levothyroxine) and under close follow-up. On follow-up, the patient's thyroid function status revealed resistance to exogenous thyroid hormone. Discussion: Thyroid agenesis is the complete absence of the thyroid gland. Meanwhile, RTH is a hereditary disease characterized by decreased sensitivity of body tissues to thyroid hormone. Most cases of RTH are due to mutations in the gene encoding for THRß. However, recently RTH due to THRα mutations has also been reported. The presentations of RTH cases in general and with thyroid dysgenesis are quite heterogenous. Conclusion: Although the combination is exceedingly rare, thyroid agenesis can coexist with RTH.