A reduced threshold of 6 mU/l will increase the number of false positive term infants by 126%, but abnormalities of thyroid function requiring treatment will be detected. We suspect that the additional expense involved in setting a lower threshold is justified.

No infant with a normal TSH concentration on first sampling had a TSH concentration that rose above 10 mU/l on second sampling, and no infants with a normal TSH concentration on first screening are receiving long-term thyroxine treatment. This study suggests that a second sample may not be necessary with a screening threshold of 6 mU/l.

Aims: Low birth weight is associated with hypothyroidism and an adverse metabolic profile in later life. Our aim was to examine the relationship between neonatal TSH and birth weight, gestational age and sex. Methods: We compared blood spot filter paper TSH levels with birth weight, gestational age and sex in a 10% sample of infants screened for congenital hypothyroidism at a single centre in Northern England. All gestational ages were included with infants suspected of having congenital hypothyroidism excluded. Results: Data on 1,728 male and 1,662 female infants were analysed. The distribution of TSH was significantly different between males and females, with males having a higher median TSH (0.7 vs. 0.6). Correlations were shown between TSH and birth weight (ρ = –0.07, p = 0.0001) and gestational age (ρ = –0.05, p = 0.008), with the birth weight association remaining independent of gestational age. The functional form of the model suggested that higher TSH in the lowest birth weight categories was responsible for the association between TSH and birth weight. Conclusions: Low birth weight is related to neonatal TSH levels. Further research is required to assess whether this association explains the relationship between higher TSH and an adverse metabolic profile in later life.