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.

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