A girl with Williams syndrome (WS) presented with elevated thyrotropin (TSH) 
levels (7.0 microU/ml), normal free thyroid hormone concentrations, and absent 
antithyroid autoantibodies. Thyroid ultrasonography and scintigraphy showed 
hemiagenesis of the left lobe and no evidence of ectopic tissue. TSH response to 
thyrotropin-releasing hormone (TRH) injection (200 microg/mq, i.v.) was 
exaggerated and prolonged, suggesting subclinical hypothyroidism. The biological 
activity of circulating TSH was slightly below the normal range [TSH bioactivity 
(B) to immunoreactivity (I) ratio (TSH B/I) = 0.4, normal: 0.6-2.2]. These 
abnormalities are similar to those seen in patients with hypothalamic 
hypothyroidism. Thyroid function is not a recognized manifestation of WS and is 
not routinely investigated. However, abnormalities of the 
hypothalamic-pituitary-thyroid (HPT) axis and thyroid dysgenesis have been found 
in other WS cases. Genes mapping at 7q11.23, contiguous to the chromosomal 
region deleted in most WS patients, may be involved in the development of the 
thyroid gland, contributing to the complex phenotype of WS.