Introduction Thyroid hemiagenesis is defined as the absence of one lobe of the thyroid gland. It represents a rare congenital abnormality with a prevalence of 0.05 to 0.2%. The left lobe is more commonly affected than the right lobe. Thyroid hemiagenesis is incidentally found during a routine thyroid screen. Herein, we report a thyroid hemiagenesis revealed by a compensatory hypertrophy of the contralateral thyroid lobe in an adolescent girl with euthyroidism. Observation A 16-year-old girl was referred to our department for an anterior right neck mass. Her past medical history was unremarkable. On physical examination, she had a large right thyroid lobe with a complete absence of the left lobe. On biological investigations, she had a TSH level of 1.07 mIU/l (nr: 0.35–4.95) and a FT4 level of 1.09 ng/dl (nr: 0.70–1.50). Cervical ultrasound showed a complete absence of the left lobe of the thyroid and a large heterogeneous right lobe with hypervascularity. Scintigraphy confirmed the hemiagenesis of the left lobe of the thyroid gland and showed a diffuse enlargement of the right lobe with a high and uniform accumulation of the tracer. No ectopic thyroid tissue was noted. Anti-thyroperoxydase antibodies and anti-TSH receptor antibodies were negative. The patient remained euthyroid over the course of follow-up. Conclusion Thyroid hemiagenesis is commonly asymptomatic and thyroid function is usually normal. However, patient may present a goiter resulting from the development of a compensatory hypertrophy of the contralateral lobe as in our case.