Reference Intervals for total triiodothyronine (TT3), free thyroxine (FT4), and thyrotropin (TSH) by chemiluminescence immunoassay in children younger than six years old from northeastern Mexico

Background: Reference values according to age groups for each population are needed for the diagnosis and follow-up of pediatric patients with thyroid diseases. Such values are unknown for Mexican infants and children younger than six years. Objective: To determine the reference values of total TT3, FT4 and TSH by chemiluminescence immunoassay in infants and children younger than six year old in Northeastern Mexico. Material and methods: Thyroid hormone serum levels were determined by chemiluminescence immunoassay in healthy infants and children younger than six years old. Results were analyzed according to gender in seven age groups: Newborns (NB), 1 to 6, 7 to 12, 13 to 18, 19 to 23, 24 to 35, and 36 to 71 months. Results: A total of 405 infants and children were included, 209 male and 196 female, 1.6 +/- 1.4 years of age (4 days to 5.6 years). Thyroid hormones: Although there were not significant differences according to gender, in NB TSH and FT4 serum levels were higher (p = 0.001 and p = 0.000, respectively) and TT3 levels were lower (p = 0.000). Conclusions: Serum levels of TSH and TT4 were higher and TT3 lower in newborns, which has been previously reported even for other measurement methods and other populations. These results allow counting with reference values of these hormones for this region.