P1095 - Time is Brain! A New Treatment Alternative for Congenital Hypothyroidism.

Introduction:
Congenital hypothyroidism (CH) is a well-known condition occurring in approximately 1:2,000 infants and is the most common cause of preventable intellectual disability (ID) worldwide. While the incidence of true CH is unrelated to gestational age (GA), preterm infants may more often be diagnosed with CH due to physiologic perturbations in thyroid physiology with GA. Given the link between CH and ID, premature infants with suspicion for hypothyroidism are often started on thyroid hormone replacement. We describe a premature infant presenting with true CH who did not respond as expected to traditional thyroid hormone replacement due to lack of gastric feeds. Given the prevalence of feeding intolerance in premature neonates combined with the poor outcomes of untreated CH the impact on treatment selection is important to consider.

Case Description:
A 25+1 week gestational age infant was noted to have Thyroid Stimulating Hormone (TSH) elevation on newborn screen consistent with hypothyroidism. A confirmatory TSH remained elevated at 122 mIU/L (3.90 ± 2.70 mIU/L) with free T4 (FT4) equal to 0.18 ng/dL(1.45 ± 0.51 ng/dL). The infant was started on Levothyroxine therapy via oral gastric tube with rapid normalization of TSH and free T4 levels. Over the next 6 weeks minor dose adjustments were made based upon trending laboratory values.

At 76 days of life, oral gastric feeds were converted to nasoduodenal (ND) feeds as part of bronchopulmonary dysplasia management. Despite this transition, thyroid supplementation continued orally. One week later, TSH uptrended to 276 mIU/L with decreased FT4 to 0.29 ng/dL. Her record was reviewed for possible interfering feeds, medications, improper administration, or defective medication. Administration of IV thyroid replacement normalized levels. Patient restarted oral levothyroxine upon normalization of thyroid hormone levels, which again increased TSH.

Treatment:
An alternate form of thyroid hormone replacement, Tirosint, containing fewer additives and resulting in more consistent absorption across a broad range of gastric pH levels was obtained and administered. Thyroid hormone levels again quickly normalized. The observation that thyroid replacement became ineffective after transitioning feeds from gastric to duodenal, suggested that gastric acid secretion stimulated by gastric feeds is necessary for absorption of levothyroxine.

Conclusion:
ND tube feeds and hypothyroidism necessitating thyroid replacement are common occurrences in the preterm neonate population. Close monitoring of TSH and FT4 levels for patients receiving ND tube feeds is of critical importance due to the impact of thyroid hormone on neurodevelopmental outcomes, particularly in preterm neonates with additional risk factors for impaired neurodevelopmental outcomes. Physicians should be aware of the impact of gastric pH alteration on thyroid medication absorption. While this management strategy has potentially major impacts on long term outcomes in neonates, there are only a handful of case reports which describe this in the medical literature.