Endocrine disorders are substantial contributors to neonatal morbidity and mortality and, of these, congenital hypothyroidism (CH) is the most common (Kumar et al. 2009). CH is a common and preventable cause of mental retardation with an incidence of approximately 1 in 2,350 live births (Hinton et al. 2010). In Iowa, the Iowa Neonatal Metabolic Screening program (INMSP) uses thyroid stimulating hormone (TSH) to screen for CH at birth. However, TSH is highly variable among healthy newborns as well as adults, leading to false positive results in some cases. Previous studies have observed that adult TSH variability is under strong genetic regulation with estimated heritability of up to 65% (Panicker et al. 2008). Additionally, there have been multiple studies examining genetic factors associated with adult TSH levels.
TSH heritability has never been estimated in the neonatal period and we aimed to determine the heritability in neonates and compare it to heritability estimates in adults. We examined 381 twin pairs obtained from the INMSP. Heritability was estimated using multilevel mixed-effects linear regression adjusting for factors affecting TSH levels; gestational age, gender, weight and age at time of sample collection. We estimated neonatal TSH heritability to be 58% with a P-value of 2x10-5, which mirrors adult heritability estimates, and provides direct evidence for a strong genetic contribution to TSH variability at birth.
We next examined genetic factors that may contribute to the observed heritability. Genetic contribution to TSH variation has been studied extensively in adults, but not in neonates. We genotyped a population of Iowa neonates; term (n=827) and preterm (n=815), for 45 single nucleotide polymorphisms (SNPs) that we selected based on reported genetic associations with adult TSH levels from the literature, as well as other candidate genes. TSH values were obtained from the INMSP. Analysis of variance was performed to identify genetic associations with TSH concentrations. The strongest association identified was rs4704397 in the PDE8B gene (p=1.3x10-4), followed by rs965513 (p=6.4x10-4) on chromosome 9 upstream of the FOXE1 gene. Both of these SNPs met statistical significance after correction for multiple testing. Our results demonstrated for the first time two genetic associations with neonatal TSH levels that replicate findings with adult TSH levels, and these findings could have clinical implications for the early prediction of risk for adult diseases and conditions associated with thyroid hormone levels.
Finally, we aimed to identify the etiologic variants that may be responsible for the observed associations by fine mapping adjacent enhancer elements. This was done in two stages; first we sequenced 58 term neonates with TSH levels at both ends of the normal distribution to identify associated variants, then we replicated the findings in an additional population of 306 term neonates. None of the observed variants were statistically significant for an association with TSH levels, however, one of the rare variants (rs112053411) identified was detected in three neonates, all in the upper distribution of TSH levels including one infant that was later diagnosed with CH. This rare variant is worth pursuing in a larger study population and in functional studies. Together this work has advanced our knowledge of the genetic basis of TSH variation in the neonatal period, as well as provided insight into the early prediction of risk for adulthood thyroid related diseases through shared genetic associations between adults and neonates.