Normal values for the fT3/fT4 ratio: centile charts (0-29 years) and their application for the differential diagnosis of children with developmental delay

Wilpert NM; Thamm R; Thamm M; Kratzsch J; Seelow D; Vogel M; Krude H; Schuelke M

doi:10.1530/ey.22.3.7

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ESPE Yearbook of Paediatric Endocrinology (2025) 22 3.7 | DOI: 10.1530/ey.22.3.7

ESPEYB25 3. Thyroid Disorders of Thyroid Hormone Transport, Metabolism and Action (3 abstracts)

3.7. Normal values for the fT3/fT4 ratio: centile charts (0-29 years) and their application for the differential diagnosis of children with developmental delay

Wilpert NM , Thamm R , Thamm M , Kratzsch J , Seelow D , Vogel M , Krude H & Schuelke M

Int J Mol Sci. 2024 Aug 6;25(16):8585. doi: 10.3390/ijms25168585. PMID: 39201272

Brief Summary: This cross-sectional study establishes age- and sex-specific reference values for the serum fT3/fT4 ratio in individuals aged 0–29 years to aid in the diagnosis of 3 rare genetic disorders that affect thyroid hormone transport, metabolism, and action: MCT8 deficiency (SLC16A2), resistance to thyroid hormone α (THRA), and selenoprotein deficiency (SECISBP2). Using 23,522 fT3 and fT4 data points from 2 large healthy cohorts, percentile charts were generated and applied to 4 patient groups. The fT3/fT4 ratio was elevated in MCT8 and THRA patients, decreased in SECISBP2 deficiency, and normal in cerebral palsy.

The study highlights that the fT3/fT4 ratio may be more sensitive than fT3 or fT4 alone for detecting patients with THRA, SLC16A2, or SECISBP2 defects, as these individuals often have hormone levels within the normal range. It also emphasizes that children with unexplained developmental delay should not only be screened for primary and central hypothyroidism (via TSH and fT4), but also for rare thyroid hormone action defects by measuring fT3 and calculating the fT3/fT4 ratio.