Thyroid Function in Infants, Children, and Adolescents: Age-Dependent Physiology, Biochemical Patterns, and Early Markers of Pediatric Thyroid Disease: A Narrative Review

Background: Thyroid hormones are critical for neurocognitive development, linear growth, and metabolic homeostasis in early life, yet pediatric thyroid physiology differs substantially from adult patterns. Age- and assay-specific interpretation of thyroid function tests (TFTs) is therefore essential to distinguish normal developmental variation from early thyroid disease in infants, children, and adolescents.Methods: We conducted a narrative review of studies published from 1990–2025 in PubMed, Scopus, and Google Scholar that reported pediatric TSH and/or free thyroxine (FT4) reference intervals, distributions of FT4–TSH patterns, prevalence and natural history of subclinical hypothyroidism (SH), hyperthyroxinemia/hyperthyroidism, and thyroid autoantibody epidemiology. Population-based cohorts, hospital series, longitudinal studies, systematic reviews, and major guidelines were prioritized; data were synthesized descriptively into six clinically oriented tables.Results: Across large normative cohorts from Europe, Asia, and South America, TSH and FT4 show a consistent trajectory: postnatal surge, higher values in infancy, gradual decline through childhood, and convergence toward adult ranges in adolescence, with notable assay- and population-dependence. In unselected pediatric TFTs, >70–85% of results are euthyroid, while abnormal patterns cluster into a minority: SH (normal FT4, elevated TSH) in ~2–8% overall—higher in iodine-deficient, obese, or high-risk groups; overt primary hypothyroidism (low FT4, high TSH) in <1–2%; central patterns (low FT4, non-elevated TSH) and TSH-resistance phenotypes are rare and largely confined to specialized cohorts. Longitudinal data show that idiopathic mild SH (TSH <10 mIU/L, antibody-negative) normalizes or remains stable in most children, whereas SH associated with Hashimoto thyroiditis, goiter, or higher baseline TSH progresses to overt hypothyroidism in up to 30–50% over several years. True pediatric hyperthyroidism is uncommon; Graves’ disease accounts for ≈95% of cases with incidence generally <1 per 100,000 child-years, while many instances of biochemical hyperthyroxinemia represent transient thyroiditis or analytical interference rather than persistent thyrotoxicosis. Thyroid peroxidase and thyroglobulin antibodies are detected in roughly 4–12% of older children and adolescents in population surveys, with marked female predominance and rising prevalence around puberty; antibody-positive children exhibit higher rates of SH and are at substantially increased risk of progression to overt hypothyroidism. A simplified clinical algorithm integrating these patterns supports risk-stratified follow-up instead of uniform treatment. Conclusions: Pediatric thyroid assessment must account for age, assay, iodine status, and autoimmunity. Most mild TFT abnormalities are transient or adaptive, whereas persistent TSH elevation with antibodies or low FT4 identifies children at highest risk. Applying population-appropriate reference intervals and a risk-based monitoring strategy can improve early diagnosis and long-term management of pediatric thyroid disorders while minimizing overdiagnosis and unnecessary therapy.