Right Thyroid Lobe Agenesis With Contralateral Compensatory Hypertrophy: A Case Report

Thyroid hemiagenesis is a rare congenital anomaly with a marked predominance of left lobe involvement. Right lobe agenesis is an exceptionally uncommon presentation that can pose significant diagnostic challenges. We report an unusual case of right thyroid lobe agenesis with contralateral compensatory hypertrophy in a 52-year-old female patient who presented for consultation after detecting anterior cervical asymmetry and increased volume via self-palpation. The biochemical profile revealed a state of clinical euthyroidism. Cervical ultrasonography demonstrated the absence of thyroid tissue in the right bed and a compensatory left lobe with a volume of 9.5 grams, alongside the presence of a 36 mm dominant nodule (Thyroid Imaging Reporting and Data System (TI-RADS) 4A). The finding of agenesis was incidentally corroborated by cervical magnetic resonance imaging, which also ruled out ectopic tissue. Management consisted of clinical observation and close sonographic surveillance following three fine-needle aspiration biopsies with non-diagnostic results (Bethesda Category I). This case illustrates the relevance of including this dysgenesis in the differential diagnosis of asymmetric cervical masses. Recognizing this anomaly is crucial to avoid unnecessary surgical interventions, such as a diagnostic hemithyroidectomy, or erroneous diagnoses of neoplasia when evaluating single functioning lobes. Conservative management should be prioritized in asymptomatic patients with unilateral agenesis to prevent permanent iatrogenic hypothyroidism. Furthermore, the importance of long-term surveillance is highlighted due to the susceptibility of the single lobe to develop nodular changes and functional exhaustion from chronic hyperstimulation.