Thyroid hemiagenesis is a rare congenital abnormality. It is difficult to find a precise prevalence rate, but it is estimated to be between 1 in 1900 and 1 in 2675.3
A study from Belgium on 2845 normal schoolchildren established a prevalence of 0.2%.4
A large prospective study on 24 032 unselected 11–14-year-old schoolchildren from Southeastern Sicily reported a prevalence of 0.05%.5
Previous studies based on symptomatic patients and the Belgian study reported a female predominance of 3:1.3 5–7
This may be due to the fact that thyroid diseases are more frequent in women than in men. A large screening study on normal children showed slight male predominance (1.4:1) although the difference between men and women was not statistically significant.5
The left lobe is most commonly affected and accounts for 80% of cases.6–8
Association with isthmus agenesis was found in 50% of cases.8
The thyroid gland develops from invagination of the endoderm in the primitive pharynx between the first and second pharyngeal pouches immediately dorsal to the aortic sac. This duct shaped invagination takes place at 16–17 days of gestation. Although it expands ventrally with the most rapid proliferation at its distal tip, it remains attached by a stalk (thyroglossal duct) to the pharyngeal floor. It begins to expand laterally at the end of the second month, forming the characteristic bilobar structure.9
Environmental or genetic factors could affect this bilobar formation and hence result in hemiagenesis. The reported familial occurrence of thyroid hemiagenesis, the occurrence among monozygotic twins and the association with other familial thyroid malformation support a genetic predisposition.10–12
A higher prevalence in areas of iodine deficiency reported by Maiorana et al5
was of no statistical significance. A higher prevalence of hemiagenesis (10.7%) in early childhood subclinical hypothyroidism was also reported.13
Maiorana et al5
also reported a subtle thyroid function abnormality. As a result they advise a follow-up for possible development of hypothyroidism. Further studies are needed to confirm such recommendations.5
Thyroid hemiagenesis has been reported as an incidental finding with a wide range of associated pathological conditions, including hypothyroidism,13
and primary hyperparathyroidism,19
as in our case.
Ultrasonography is the diagnostic tool of choice for detecting thyroid hemiagenesis.3
Differential diagnoses of absent lobe images on thyroid scintiscan include solitary toxic adenoma, primary or secondary neoplasms, infiltrative diseases, unilateral inflammations and true hemiagenesis. As a result, ultrasonography is not considered the best imaging modality for hemiagenesis.11
In our case surgery was already indicated and hemiagenesis confirmed at the time of operation.
- Thyroid hemiagenesis is a rare congenital abnormality.
- The left lobe is most commonly affected.
- Ultrasonography is the diagnostic tool of choice.
- Lobectomy, for possible cancer, becomes total thyroidectomy in such cases.
- Thyroid hemiagenesis prepared for thyroid lobectomy should be warned about the inevitable postoperative hypothyroidsm.