Cortisol and thyroid hormones are critical to normal fetal development and neonatal transition, and baseline values and stimulation tests are abnormal after preterm birth. To evaluate cortisol and thyroxine (T4) responses that are not influenced by uncontrolled antenatal events associated with human preterm labor, we measured cortisol and T4 after standard-dose adrenocorticotropin (ACTH) and corticotropin-releasing hormone (CRH) stimulation tests, as well as high-dose CRH and thyrotropin-releasing hormone stimulation tests in baboons that were delivered for 3 separate protocols at 125 days of gestation (term is 186 days). The animals were surfactant treated and ventilated for up to 14 days. Some fetuses were exposed to fetal or maternal betamethasone, and some newborns were treated with 10 µg/kg T4 for 9 days after birth. Baseline cortisol levels were in a stress range of 30–60 µg/dl by day 5. Cortisol did not increase consistently until day 11 in response to a high CRH dose or ACTH. T4 treatment for 9 days after birth suppressed the cortisol responses and subsequent baseline T4 levels. The hypothalamic-pituitary-adrenal (HPA) axis was unresponsive to standard dose stimulation tests until 11 days of age in preterm baboons, indicating HPA immaturity.

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