OBJECTIVE: To verify the prevalence of morpho-volumetric and functional thyroid 
abnormalities in young patients with Williams syndrome (WS).
STUDY DESIGN: Ninety-two patients with WS (49 boys and 43 girls, 0.2-17.2 years 
of age) underwent evaluation of thyroid function by means of thyroid-stimulating 
hormone (TSH), fT3, and fT4 measurement. Thyroid ultrasonography was performed 
in 37 patients. Thyroid antibodies (thyroid peroxidase and thyroglobulin) were 
measured in all patients with abnormal thyroid function tests.
RESULTS: None of our patients had overt hypothyroidism; 29 patients (31.5%) had 
subclinical hypothyroidism. Thyroid antibodies were absent in all patients. The 
prevalence of patients with subclinical hypothyroidism was significantly higher 
in the younger patients. Ultrasonography revealed morphological or volumetric 
abnormalities of the thyroid gland in 67.5% of patients; these abnormalities 
were more frequently observed in the older children.
CONCLUSIONS: Subclinical hypothyroidism is a frequent but stable finding in 
young children with WS. The great majority of patients with WS >10 years, either 
with normal or hypoplastic thyroid, have normal thyroid function. Therefore, we 
suggest yearly monitoring of thyroid function and sonographic studies at least 
once in patients with WS. Treatment should be reserved for the patients with 
overt hypothyroidism or for those whose thyroid function shows signs of 
progressive deterioration.