Isolated hypothyroxinemia (IH) is defined as a thyroxine level in the lower 5th (severe IH) or 10th percentile (mild IH) of the pregnancy-related reference range and a normal TSH. The etiology of IH remains unknown. This review aims to evaluate the biochemical criteria used to define IH in different published studies and to discuss potential maternal as well as fetal outcomes and whether treatment during early pregnancy can prevent the eventual adverse effects. For the current literature a better standardization of free thyroxine assays is needed, as well as the use of appropriated trimester-specific reference intervals for thyroid function tests. Today no study demonstrates a benefit from treating early pregnant IH women on perinatal and fetal outcomes.

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The prevalence of hypothyroxinaemia in early pregnancy was of 8·7%. IH is associated with an increased maternal BMI and is related with a risk of breech presentation, a significant increase in macrosomia and caesarean sections. Screening should consider overweight as risk factor for hypothyroxinaemia.

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