Estado tiroideo y hemodinámico en el recién nacido de peso extremadamente bajo evaluación sistemática del impacto de la función tiroidea y el suplemento hormonal precoz sobre la circulación sistémica y cerebral

Abstract

Background: Transient hypothyroxinemia is common among infants born before 30 weeks gestation and associates neurodevelopmental impairment. Thyroid hormone supplementation (THS) given early during the neonatal period to the extremely low birth weight (ELBW) infant could be beneficial. However, thyroid hormones play an important role on the cardiovascular system that has not been systematically evaluated. Objective: To explore the thyroid hormones serum levels profile in the ELBW infant and to determine the impact of thyroid hormone status and THS on the systemic and cerebral hemodynamics. Methods: We conducted a prospective cohort study encompassing two periods of time at a tertiary neonatal unit. During the first period-2005/2006-, thirty three infants less than 28 weeks gestation pertaining to the international THOP Supplementation Study (code 1 R01 NS45109-01 A1), a randomized, blind clinical trial, were enrolled. Patients were assigned to 6 different intervention groups, four of them including combined administration of triiodothyronine (T3) -1 ¿g/kg/d, 14 days- and tetraiodothyronine (T4) -8 y 16 ¿g/kg/d, 6 weeks; bolus or continuous infusion-. During the second period -2009/2010-, twenty two consecutively born infants of similar clinical characteristics were enrolled, none of them receiving THS. Serial thyroid hormone serum levels, structural and functional echocardiography including superior vena cava flow (SVCF) estimation, and cerebral near infrared-spatially resolved spectroscopy monitoring (NIR-SRS) were performed during the first 8 weeks of postnatal life. Results: T3 and T4 combined administration induced suppression of TSH. Both, heart rate and cardiac output were higher in patients on THS as compared to those without. The hemodynamic effects of THS were observed shortly after intervention started. Developmental profile of cardiac output and SVCF on the one hand, and vascular resistance on the other, were positively and negatively associated with T3 serum levels, respectively. A decrease in cerebral tissue oxygenation index and an increase in fractional oxygen extraction were observed during the first week irrespective of intervention. Beyond this time, this pattern persisted only in infants receiving THS. THS also showed acute effects on cerebral circulation that appeared shortly after start of treatment but differed according to mode of administration ¿bolus or continuous infusion-. Conclusions: According to our study protocol, THS causes changes in systemic and cerebral circulation in the ELBW infant. Trends in systemic hemodynamics mirror those of T3 serum levels. Mode of T4 administration is determinant of its effects on cerebral circulation-oxygenation. Key Words: extremely low birth weight infant, hypothyroxinemia, euthyroid sick syndromes, thyroxine, hemodynamics, echocardiography, near-infrared spectroscopy.