Hypothyroidism in Thyroid Hemiagenesis: A Case Report.

Thyroid hemiagenesis is one of the rare developmental abnormalities of the thyroid. It is more common in women and more commonly found on the left side, often associated with the absence of isthmus. In most instances, thyroid hemiagenesis is presented with hyperthyroidism. This case report aimed to present a 72-year-old female who presents with thyroid hemiagenesis and associated hypothyroidism. She was initially diagnosed with hypothyroidism about 25 years ago. There was no family history of thyroid disease, and she never underwent any neck, thyroid, or parathyroid surgeries. Her most recent laboratory investigations revealed thyroid-stimulating hormone level of 0.93 mIU/ml, free tetraiodothyronine of 0.93 mcg/dl, free triiodothyronine of 2.75 ng/dl, anti-thyroid peroxidase of 2.0 IU/ml, and thyroid-stimulating immunoglobulin of less than 1.0 IU/l. An ultrasound study of her neck revealed an absence of the left thyroid lobe and isthmus. The diagnosis of congenital thyroid hemiagenesis with hypothyroidism was made, and the current treatment with 75 mcg of levothyroxine was continued. Thyroid hemiagenesis is an incidental finding and may present later in life. Thyroid hemiagenesis is commonly associated with hyperthyroidism, but it can present with hypothyroidism. Patients with thyroid hemiagenesis may be at higher risk for developing hypothyroidism than their normal counterparts due to smaller thyroid hormone reserves.