In the Williams-Beuren syndrome (WBS), disorders of the thyroid function and 
morphology have been reported and programs of thyroid screening and surveillance 
are recommended. However, the frequency of biochemical thyroid assessment, 
particularly in the first year of life, is being debated. In this report we 
describe an infant with WBS and congenital hypothyroidism, due to an important 
thyroid hypoplasia. The patient, a 1-month-old female, negative at primary 
neonatal thyroid screening, was referred to our hospital for dyspnea. Thyroid 
function tests showed a raised TSH (42 mIU/l; normal range 0.5-4 mIU/l) with a 
low FT(4) concentration (10.21 pmol/l; normal range: 10.29-24.45 pmol/l). 
Ultrasound examination of the neck showed a significant thyroid hypoplasia, 
whereas (99m)Tc-pertechnetate thyroid scintigraphy evidenced a thyroid gland in 
normal position, with reduced shape and overall weak fixation. Therefore, 
treatment with L-thyroxinewas started. Thyroid hypoplasia is a frequent 
characteristic of WBS and abnormalities of thyroid function are common in 
patients with this feature. Therefore, the possibility of congenital 
hypothyroidism should always be taken into consideration too and, even if 
congenital hypothyroidism neonatal screening is negative, thyroid (morphology 
and function) evaluation should be regularly assessed when the diagnosis is made 
and, thereafter, every year in the first years of life.