The authors report a female presenting with congenital heart defects, liver 
hemangiomas, and facial dysmorphisms admitted to hospital at 3 months of age 
because of feeding difficulties and poor growth. She had hypotonia and large 
tongue, "coarse" face, and umbilical hernia in presence of complex congenital 
cardiovascular malformations. In spite of normal neonatal screening we performed 
serum levels of thyroid hormones. Thyrotropin level was very high (>50 
microU/ml; normal value 0.2-4 microU/ml), while serum free T(3) (FT3) and free 
T(4) (FT4) levels were normal (FT3 3.6 pg/ml, normal value 2.8-5.6 pg/ml; FT4 
11.6 pg/ml, normal value 6.6-14 pg/ml); antithyroid autoantibodies were absent. 
Thyroid scintigraphy with sodium 99m Tc pertechnetate showed a small ectopic 
thyroid located in sublingual position, so treatment with L-thyroxine 37.5 
microg/24 hr was started with rapid improvement of the clinical picture. At 17 
months of age the patient developed the complete characteristic phenotype of 
Williams syndrome (WS); the clinical diagnosis was proven by fluorescent in situ 
hybridization (FISH) analysis which showed hemizygous deletion of the elastin 
gene on chromosome 7. Recently a case of thyroid hemiagenesis in a child with WS 
has been reported; our patient underscores the association of hypothyroidism and 
WS. Moreover, our case shows that clinical manifestations of hypothyroidism may 
be present and the treatment may be necessary as it is in isolated congenital 
hypothyroidism.