Effects of Maternal Iodine Deficiency in Women with Mild Thyroid Dysfunction During Early Pregnancy on Mental and Motor Growth of Their Offspring During Levothyroxine Treatment

The relationship between maternal thyroid function and intellectual development of offspring is controversial. Iodine may be an important confounding factor. This study investigated whether maternal iodine status could affect the efficacy of levothyroxine (LT4) treatment during early pregnancy on the intellectual growth of progeny.This prospective study divided participants into two groups; the normal iodine group included 53 mother-child pairs and the low iodine group included 60 mother-child pairs (urinary iodine concentration (UIC) ≥ 150 µg/L and UIC < 150 µg/L). Each iodine status group was further subdivided according to specific maternal thyroid disorders. The two groups were categorized as follows: Control(N), hypothyroxinemia (IH) + LT4, subclinical hypothyroidism (SCH) + LT4, positive thyroid peroxidase antibodies (TPOAbs) + LT4. Finally, the study included eight groups. The Bayley Scales of Infant Development-II were employed to evaluate the neurodevelopment of children (age: 12 -30 months). The main results were age-adjusted scores from the Mental Development Index (MDI) and Psychomotor Development Index (PDI).We found that: 1) Under the similar conditions of thyroid function and treatment, iodine deficiency during early pregnancy reduced the MDI value of the offspring (P < 0.001), while, the PDI value was not affected (P = 0.276). Linear regression demonstrated a substantial positive correlation between maternal UIC and MDI of offspring (B = 0.09 [CI 0.06-0.13]; P = 0.01). In the case of iodine deficiency during pregnancy, LT4 treatment on SCH and IH could not improve offspring MDI scores (P < 0.001, P = 0.037, respectively) in contrast to the normal group. However, under normal iodine status during pregnancy, LT4 treatment on SCH and IH could improve offspring MDI scores (P = 0.525, P = 0.650, respectively) compared with the normal group. Concurrently, the PDI of the aforementioned categories did not differ significantly (P > 0.05). 3) In comparison to the normal group, LT4 treatment on TPOAbs during pregnancy could not improve the MDI of the offspring, regardless of whether the iodine nutritional status was normal or not (P < 0.001, P = 0.047, respectively). These findings suggest that concurrent assessment and optimization of both thyroid function and iodine status during early pregnancy may be essential for maximizing offspring intellectual development.