The study found no relationships between serum vitamin D levels in infants and the prevalence of Neonatal Indirect Hyperbilirubinemia (NIH). Although the mean serum vitamin D level was lower in the infants with NIH compared to the healthy infants, no significant differences were observed between the two groups and serum vitamin D level was not significantly related to the prevalence of NIH [
15]. About two-thirds of newborns develop clinical neonatal jaundice (serum bilirubin level > 5 mg/dl) and more than 97% of term and preterm infants develop biochemical hyperbilirubinemia (serum bilirubin level > 1 mg/dl); [
16,
17]. Clinical guidelines recommend identifying the causes of hyperbilirubinemia and adopting effective prevention strategies [
18]. Identifying the treatable etiology of hyperbilirubinemia and preventing its prevalence, which comprised the objectives of the present study, are essential. A similar prospective study by M. Mutlu et al. [
19] investigated the relationship between vitamin D level and neonatal hyperbilirubinemia in term infants and compared vitamin D levels in infants with pathologic hyperbilirubinemia and healthy infants with normal or physiological levels of bilirubin. The results showed statistically significant differences between the control and case groups in terms of 25-hydroxyvitamin D levels (
P = 0.01). A significant negative relationship was also observed between vitamin D levels and the parathyroid hormone in the infants (
P = 0.03). Mutlu’s study was conducted over 1 year on infants aged three to ten days and born at a gestational age of 37–40 weeks who had a serum bilirubin level requiring phototherapy (group 1 or the case group) and healthy infants of the same age but without jaundice or with physiological jaundice only (group 2 or the control group). Both groups were examined at identical time periods in terms of their birth weight, gestational age, neonatal age, weight at the visit, type of delivery, gender, type of nutrition, mother’s age, mother’s type of clothing, place of residence (geographical region), vitamin D supplementation during pregnancy, the mother’s disease history and the mother’s medication history during the pregnancy, which could affect the level of vitamin D. Infants born to mothers with symptoms of chronic liver disease and kidney disease or those who regularly used anticonvulsants were excluded from the study [
19]. Just as in the discussed study, the present study considered all these criteria, with few differences, including examining infants with a gestational age of 37 to 42 weeks and following up on all the infants in the control group until 15 days and eliminating the cases of physiological jaundice from the control group, such that even suspicions about very mild jaundice only in the sclera meant exclusion from the study. The groups were also homogenized in terms of their demographic variables and socioeconomic status. A study by E. Dan-Ierodiaconou et al. (1980) on the effect of phototherapy on vitamin D metabolism examined ten infants with jaundice under phototherapy treatment. The infants’ 25-OHD, 24, 25(OH)2D, Ca and P levels were measured before phototherapy and 24 and 48 h after the procedure was over. The mean weight of the infants was 3.4 kg, their mean age 64 h, their mean bilirubin level was 17.1 mg and their mean duration of phototherapy 83 h. The causes of jaundice included G6PD deficiency, Rh incompatibility, blood type incompatibility and unknown factors. For their phototherapy, the infants were placed under seven lamps positioned 60 cm above them and were fed with cow’s milk containing no vitamin D. The results of the study showed that the skin of infants with jaundice treated with phototherapy does not convert pro-vitamin D to active vitamin D [
20‐
22]. A similar study by Gillies DR et al. (1984) on the effect of phototherapy in infants with neonatal jaundice on the production of vitamin D measured the 25(OH) VitD level before and 48 h after phototherapy. The results of the study showed no significant increase in active vitamin D levels 48 h after phototherapy [
23]. Given the disparities in the results of the discussed studies and the potential impact of phototherapy on serum vitamin D levels, blood samples from the case group were sent to the laboratory for measuring the infants’ serum vitamin D level before starting phototherapy in the present study. In Mutlu’s study, hemogram, peripheral smear, reticulocyte count, blood group, direct Coombs, bilirubin, free T4, TSH, Ca, P, Mg, ALP, PTH and 25(OH) VitD testing was performed on all the infants. Ca, P, Mg, ALP, PTH, 25(OH) VitD and blood group testing was performed on all the mothers too. Given the existing imitations, the present study measured only Ca, P, Mg, ALP and 25(OH) VitD in the infants and 25(OH) VitD in the mothers. The objective of the present study was to evaluate the relationship between hyperbilirubinemia and serum vitamin D levels –not to assess the etiology of vitamin D deficiency; as a result, PTH, TSH and free T4 were not measured in this study. Considering the exclusion of infants with physiological icterus, the total and indirect bilirubin levels were not measured in the control group. Hemogram, peripheral smear, reticulocyte count, blood group, direct Coombs and G6PD activity testing were performed in all the icteric infants in the case group and the cases of hemolytic icterus, as diagnosed by the neonatologist, were excluded from the study. Mutlu’s research was a case-control study with a one-on-one design that was conducted on 51 infants, since some of the families that were entered into the study withdrew, and 30 infants with hyperbilirubinemia ultimately remained in group 1 and 21 healthy infants remained in group 2. In terms of laboratory parameters, bilirubin levels were significantly higher in group 1 compared to in group 2, while there were no statistically significant differences between the two groups in terms of Ca, P, Mg and ALP in the infants and Ca, P, Mg, ALP, PTH and 25(OH) VitD in the mothers. There was a significant difference between the two groups in terms of serum 25-hydroxy vitamin D levels and a significant negative relationship was also observed between vitamin D levels and PTH. Vitamin D deficiency was reported in 86% of the infants in group 1 and vitamin D inadequacy was reported in 7%. The case group had a significantly more severe degree of vitamin D deficiency compared to the controls, but there were no significant differences between the two groups in terms of vitamin D inadequacy. In the present study, none of the participants withdrew from the study because the parents were given adequate explanations, the physician visits were free and the families incurred no additional costs. The findings of this study showed a statistically significant difference between the control group and the infants with hyperbilirubinemia in terms of 25-hydroxyvitamin D level. There was also a significant negative relationship between vitamin D level and the parathyroid hormone in the infants. This disparity could be due to the high prevalence of vitamin deficiency D among Iranians, which led to an inadequate mean serum vitamin D level in both groups.