Repeat testing for congenital hypothyroidism in preterm infants is unnecessary with an appropriate thyroid stimulating hormone threshold

Abstract

Background: Revised UK neonatal screening guidelines recommend that a second blood sample for assay of thyroid stimulating hormone (TSH) be taken when preterm infants reach a postmenstrual age of 36 weeks.

Objective: To examine the results of a regional screening programme to see whether a rise in TSH concentration was observed in some preterm infants between the first sample taken around 5 days after delivery and the second sample taken at around 36 weeks.

Methods: Whole-blood TSH concentrations in preterm infants born over a 2-year period (April 2005 to March 2007) were assessed, and the number of infants in whom there was a fall or rise to values below or above the local screening threshold (6 mU/l) was determined.

Results: Baseline TSH samples were obtained from 2238 preterm infants (median gestational age 32 weeks, range 21-35) with second samples obtained from 2039 (median gestational 32 weeks, range 23-35). In 19 infants, TSH concentrations fell from above to below the screening threshold, and in five infants values rose from below the screening threshold to 6-10 mU/l. However, TSH concentrations fell to <6 mU/l on a further blood spot in four of these infants, and the remaining infant had a serum TSH of 6.8 mU/l. Three infants had raised TSH concentrations on both occasions with unequivocal hypothyroidism (serum TSH >80 mU/l). The initial TSH concentration in one of these infants was 6-10 mU/l.

Conclusions: 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.