Background: Hemiagenesis of the thyroid is a rare congenital condition that is characterized by an absence of a thyroid lobe. The prevalence rate of hemiagenesis is about 0.02% and is usually detected incidentally. Hemiagenesis of the thyroid gland is a rare congenital anomaly with about 800 cases reported in the literature untill 2020. Case description: A 36-year-old female presented with complaints of weight loss, palpitation, and tremors for the past 3 months. She also gives a history of right-sided neck swelling of a duration of 1 month. A thyroid function test was done and revealed a hyperthyroid state (free T₄:3.2 ng/dL, free T₃:12.7 pg/mL, TSH: <0.005 mIU/mL). Ultrasound revealed a diffusely enlarged (4.5 × 3.5 × 3 cm) heterogenous hypoechoic right lobe with increased vascularity on Doppler and isthmus of thickness 7.9 mm and an absent left lobe. Thyroid scintigraphy revealed an increased uptake of 8% in the right lobe and isthmus with absent uptake of tracer in the left lobe. A thyroid antibody panel was done and it revealed elevated levels of anti-thyroid stimulating hormone (TSH) receptor antibodies and anti-thyroid peroxidase (TPO) antithyroid microsomal antibody (AMA) antibodies. She attained a clinical and biochemical euthyroid state after 3 months of medical treatment and was admitted and planned for total thyroidectomy. The final histopathology report was suggestive of hyperplastic goiter with an absent left thyroid lobe. Conclusion: Thyroid hemiagenesis is a rare congenital anomaly of the thyroid gland and when suspected the diagnosis should be confirmed by an ultrasound neck and a thyroid scintigraphy scan. Clinical significance: Patients with hemiagenesis of the thyroid have a higher risk of developing thyroid disorders and should be kept under close follow-up to diagnose them early for a better outcome.