MON-367 - Graves' Disease in A Solitary Thyroid Lobe: A Rare Case of Right-Sided Thyroid Hemiagenesis

Introduction: Thyroid hemiagenesis is a rare congenital anomaly characterized by the complete absence of one thyroid lobe, with or without absence of the isthmus. Its estimated prevalence ranges from 0.02% to 0.2% in the general population and is most often identified incidentally during evaluation for thyroid disease. The left thyroid lobe is most commonly affected, while right sided hemiagenesis is particularly rare. Coexisting thyroid disorders are frequently observed in patients with thyroid hemiagenesis, with up to 40% demonstrating abnormal thyroid function. Here, we describe a case of Graves’ disease occurring in a patient with right thyroid hemiagenesis.
Case report: A 44-year-old woman with no known past medical history presented with bilateral lower-extremity swelling, palpitations, and shortness of breath. She reported an unintentional 10-pound weight loss over the preceding two months, accompanied by heat intolerance, anxiety, and tremors. Family history was significant for thyroid disease. On physical examination, the patient appeared restless and tachycardic, with bilateral fine hand tremors. There was no evidence of proptosis, lid lag, or thyroid stare. The left thyroid lobe was slightly enlarged, mobile with swallowing, non-tender, and without bruit or surgical scars. Laboratory evaluation revealed a suppressed TSH < 0.05 µU/mL (0.30–4.2), elevated free T4, 3.1 ng/dL (0.6–1.6), and elevated T3, 315 ng/dL (60–160), consistent with thyrotoxicosis. The patient was started on beta-blocker and methimazole. Thyroid autoantibodies, including thyroid peroxidase (TPO), thyroid-stimulating immunoglobulin (TSI), and thyrotropin-binding inhibitory (TBI) antibodies, were positive, supporting the diagnosis of Graves disease. Thyroid ultrasound demonstrated a mildly enlarged left thyroid lobe measuring 6.9 × 2.6 × 3.2 cm with heterogeneous echotexture and increased vascularity, findings consistent with autoimmune thyroid disease. The right thyroid lobe and isthmus were not visualized. In the absence of prior neck surgery, radiation, or trauma, these findings raised concern for congenital thyroid hemiagenesis.

Conclusion: Thyroid hemiagenesis is a rare congenital anomaly that occurs more frequently in females, predominantly involves the left thyroid lobe, and is thought to have a genetic background. It is most often identified incidentally in the setting of coexisting thyroid pathology. This case illustrates the uncommon coexistence of Graves disease in a patient with right-sided thyroid hemiagenesis and underscores the importance of maintaining a high index of suspicion for congenital thyroid anomalies when asymmetric thyroid imaging findings are encountered. High-resolution ultrasonography is the preferred diagnostic modality for identifying thyroid hemiagenesis, as it is widely available and cost-effective.

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