Lack of adult-type salivary cortisol circadian rhythm in hospitalized preterm infants.

Background/aims: Knowledge of the presence or absence of cortisol (F) circadian rhythm in preterm infants is important for the interpretation of F measurements made in samples taken for both clinical and research purposes. Little is known about its emergence in very preterm infants. This study examines circadian rhythm in F secretion in hospitalized infants born before 30 weeks' gestation.

Design: Prospective longitudinal observational study.

Subjects: 11 infants admitted consecutively and born before 30 completed weeks of gestation.

Measurements: F was measured by highly specific radioimmunoassay on morning and evening saliva samples gathered at weekly intervals until discharged home. Circadian rhythm was defined as > or =40% reduction from morning to evening level.

Results: For all data, the median salivary F was 10.3 nmol/l (range <0.5-372.8). F levels were highest in the first 3 weeks of life. No infants displayed classical circadian rhythm for 4 weeks or more prior to being discharged from hospital. The other infants showed randomly distributed morning and evening F values with a trend in 4 infants towards periods of consistently higher evening than morning values.

Conclusion: Adult-type F circadian rhythm is rarely evident in hospitalized preterm infants born before 30 weeks' gestation.