Lower limit of free thyroxine reference interval in neonates to choose therapeutic tactics for congenital hypothyroidism

OBJECTIVES: Congenital hypothyroidism (CH) is one of the common thyroid disorders in children. Early detection and prompt initiation of hormone replacement therapy (HRT) are important for further normal neonatal development. In clinical practice, one of the key problems is the differentiation of fT4 level to identify cases of obvious hypothyroidism requiring immediate prescription of levothyroxine, and subclinical hypothyroidism, allowing the clinician to understand the causes of elevated TSH during dynamic monitoring and make a cardinal decision on the need for lifelong HRT.

AIM: To establish the values of the lower limit of the reference interval (LLRI) for fT4 in newborns to decide on therapeutic tactics.

MATERIALS AND METHODS: Until 2013 laboratory tests were performed on a «Wallac DELFIA» analyser (normative values for fT4 9.8–16.8 pmol/l), then on a «bioMerieux VIDAS» analyser (normative values for fT4 10.6–19.4 pmol/l). RI was determined based on the results of cT4 measurements in newborns examined in the neonatal screening framework at the clarifying diagnosis stage.

RESULTS: LLRI for fT4 of all 1596 neonates was 10.9 pmol/l (90% CI 10.5; 11.1), in full term neonates — 11.10 pmol/l, (90% CI 10.70–11.60 pmol/l); in premature neonates — 9.63 pmol/l (90% CI 9.20–11.30 pmol/l).

ONCLUSIONS: in clinical practice, it is reasonable to use the cut-off point for fT4 below 11.0 pmol/l to decide on the need for HRT, irrespective of the day of clarifying diagnosis and gestational age. If the level of fT4 is higher than 11.0 pmol/l, regardless of the level of TSH increase, it is possible to interpret this condition as subclinical hypothyroidism and to choose the tactics of dynamic follow-up with monthly control of thyroid tests. 

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