Right thyroid lobe agenesis and left thyroid colloid benign nodule discovered incidentally in female with breast carcinoma receiving chemotherapy for multiple metastases: Case report and review of the literature

Introduction and importance: Thyroid hemiagenesis (THA) is the failure of embryologic development of a lobe of the thyroid gland and is a rare anomaly of uncertain incidence. The left lobe is more commonly absent than the right lobe. It is discovered incidentally during investigations. Case presentation: A 48 year old Egyptian female presented at the thyroid surgery clinic at our institution to follow up after a nodule left thyroid lobe accidently discovered on positron emission tomography (PET) scan undertaken to follow up on bone metastasis of breast cancer which was surgically removed about 14 years ago. Clinical discussion: The patient looked clinically well with no scar in the anterior of the neck, no palpable thyroid nodules, and no lymphadenopathy. Ultrasound imaging of the neck revealed absent right thyroid lobe tissue and a nodule was noted at the upper pole of the left thyroid. Laboratory tests unremarkable, with TSH (2.14 mIU/L), and FT4 (12.4 pmol/L) within normal range. Fine needle aspiration and cytology of the thyroid nodule revealed atypia of undetermined significance. Conclusion: THA is rare and right THA is even rarer. It is usually asymptomatic, and diagnosis is mostly incidental while investigating symptoms due to pathology of the other thyroid lobe or any of the parathyroid glands. In much rarer circumstances, right THA might be discovered when investigating conditions not related to the thyroid or parathyroid glands years after the initial pathology as in the current case. Etiology is inconclusive but genetic factors could play a role. No treatment is required if no symptoms are present.