Neonatal screening for hypothyroidism in Scotland: results of a pilot study.

We have assessed the feasibility of screening for neonatal hypothyroidism based on thyroxine (T4) or thyrotrophin (TSH) assays on dried blood spots collected for the phenylketonuria (PKU) screening programme in Scotland. The TSH assay was both more sensitive and specific than T4 in detecting primary hypothyroidism and was sufficiently rugged and rapid for routine application. Using a labelled antibody assay for TSH, 99.7 per cent of the first 30,000 babies tested had values less than 25 mU/l, and 0.24 per cent had values between 25 and 50 mU/l. Of the 11 suspected positives (TSH greater than 50 mU/l), nine were confirmed on serum specimens as primary hypothyroidism (prevalence 1:3,300) and the two false positives (0.007% of total) were explicable by inappropriate time of sampling. No false negatives have so far been identified. The median time to detection by blood spot assay was 16 days (range 6 to 22 days). Five of the nine cases were not suspected clinically at the time of screening. It is concluded that TSH fulfils most of the criteria for an ideal screening test for primary hypothyroidism and a screening programme based on blood spot TSH will allow therapy to be commenced within one month of birth.