Effect of phototherapy on thyroid stimulatory hormone and free thyroxine levels

Abstract

Objective: To study the effect of phototherapy for neonatal hyperbilirubinaemia on thyroid function as neonatal thyroid screening is sometimes performed during exposure to phototherapy. Methodology: Infants with non-haemolytic hyperbilirubinaemia were sequentially allocated to fibre-optic phototherapy, conventional daylight phototherapy, or a combination of both. Bilirubin concentration was monitored 12 hourly by capillary blood sampling; venous blood was sampled for thyroid stimulating hormone (TSH) and free thyroxine (fT 4) determinations, at start of exposure, at 24 h, end of exposure and 1 day later. Comparable unexposed infants served as controls. Results: All 123 study infants and 25 controls remained well during the study. Bilirubin levels declined during phototherapy, being most rapid in the combination group. The TSH and fT 4 values at start of exposure were 3.86 ± 0.41 mU/L (mean ± SEM) and 33.20 ± 1.16 pmol/L, respectively, in the fibre-optic group, 3.62 ± 0.38 mU/L and 37.22 ± 1.76 pmol/L in the daylight group, and 4.40 ± 0.48 mU/L and 29.91 ± 1.13 pmol/L in the combined group, compared with 5.77 ± 0.40 mU/L and 34.46 ± 1.68 pmol/L in the control group. The TSH and fT 4 values declined with increasing age in the phototherapy and control groups with end of exposure values of 2.90 ± 0.28 mU/L and 27.71 ± 0.71 pmol/L, 2.77 ± 0.31 mU/L and 33.52 ± 1.22 pmol/L, and 3.44 ± 0.30 mU/L and 27.54 ± 0.88 pmol/L, respectively, compared with 4.21 ± 0.61 mU/L and 27.19 ± 2.33 pmol/L (at 72 h) in the control group. The pattern of TSH and fT 4 decline in the exposed and control groups was similar, being related to increasing age. Conclusions: The validity of neonatal thyroid screening is not affected by fibre-optic or conventional phototherapy or by both combined.