Objective. Long-term outcome of thyroid function in children with very short lasting neonatal hyperthyrotropinemia ("false positive" at neonatal screening) was studied in an observational, prospective study.

Thyroid function and morphology were evaluated in 44 "false positive" children up to advanced childhood (8.0±0.7 years of age). In these children a high prevalence (50%) of subclinical hypothyroidism in early childhood (2.8±0.5 years) had already been described.

Results. At average 5.3 years subclinical hypothyroidism persisted in 19/44 (43.2%) children and, more specifically, in 2/3 of those who had increased TSH in early childhood. Euthyroidism was present in all cases that were euthyroid in early childhood, although they had TSH and FT3 values significantly higher than control children with a normal TSH at birth (TSH= 2.6±0.7 vs 1.5±0.6 mU/L, p<0.001; FT3= 4.9±0.8 vs 3.9±0.9 pmol/L, p<0.01). Thyroid morphology alterations were frequent in the group of children with subclinical hypothyroidism.

At average 8.0 years, subclinical hypothyroidism persisted in 14/44 (31.8%) children. In all other children TSH and thyroid hormones were confirmed within the normal range.

Conclusions. This prospective longitudinal study confirms that newborns "false positive" at neonatal screening have a high risk to develop persistent subclinical hypothyroidism. The prevalence of hypothyroidism decreases with increasing age, but it is still high (over 30%) in late childhood. Even those "false positive" children that maintain euthyroidism in late childhood have an average TSH value that, although within the normal range, is higher than in normal controls, a possible marker of minor congenital thyroid function abnormalities.