In a study of over 7,000 thyroid scans, unilateral agenesis was found in only four patients. 4 Another study involving 12,456 patients who underwent thyroidectomy reported seven cases of thyroid hemiagenesis. 5 Unilateral agenesis is reported to be more common in the left side, and in females. 6,7 The patient in this report, although a female, had a right-sided hemiagenesis. Most cases of thyroid hemiagenesis are discovered when patients present with a lesion in a functioning lobe.
Clinical examination is of little help in diagnosing hemiagenesis.
When the isthmus and the lobe are both absent, tracheal rings may be easily palpable and the edge of sternomastoid on the affected side is nearer the midline overlying the trachea. In the present case, although the isthmus and the right lobe were absent, these physical signs were not appreciated preoperatively. Absence of uptake on the affected side in a thyroid scan may raise suspicion, but usually it may be passed over as a suppressed lobe by an overactive enlarged lobe, or big cold nodule. TSH-stimulated thyroid scan has been used to differentiate between thyroid hemiagenesis and a suppressed lobe. It will show the total absence of uptake in congenital hemiagenesis, but activity will reappear in the suppressed lobe.