Hypothyroidism in preterm infants following normal screening.

Area of Science:

• Pediatrics
• Endocrinology
• Neonatology

Background:

• UK newborn screening uses blood spot thyroid-stimulating hormone (TSH) at 5-8 days.
• Repeat screening for preterm infants at 36 weeks' gestational age was introduced in 2005.
• Variations exist in TSH cut-off values used across screening programs.

Observation:

• Two preterm infants with initially normal TSH screening values later became screen-positive upon re-testing at term.
• Case 1 (born at 29+6 weeks, with Trisomy 21) showed a TSH rise from 3.3 mU/L to 263 mU/L (plasma 476.5 mU/L) at term-corrected age.
• Case 2 (born at 24+6 weeks) had TSH <2 mU/L initially, rising to 6.4 mU/L at 36 weeks corrected, and 66.6 mU/L (with low free thyroxine) after a barium enema.

Findings:

• Preterm infants' hypothalamo-pituitary-thyroid axis immaturity complicates congenital hypothyroidism screening.
• Intercurrent illnesses and medications can significantly affect thyroid function in neonates.
• These cases highlight potential risks associated with not re-screening ex-preterm infants at term.

Implications:

• The findings suggest that current screening protocols may not adequately identify all cases of congenital hypothyroidism in preterm infants.
• Re-screening ex-preterm infants at term may be necessary to ensure timely diagnosis and treatment.
• Further research is warranted to optimize screening strategies for congenital hypothyroidism in vulnerable preterm populations.