Impact of iodine deficiency on thyroid function in pregnant African women - A possible factor in the genesis of 'small for dates' babies

The effect of iodine deficiency (ID) on thyroid function in African women during pregnancy and postnatal period was examined, for which very limited information is available. Serum T4, T3, TSH, TBG, thyroxine binding capacity (TBK), free thyroxine index (FT4I=T 4TTBK) and T4/TBG were determined by ELISA technique in 32 pregnant women (resident of Plateau state, Nigeria - an ID zone) through pregnancy upto term delivery, and in 5 women up to 6 weeks post delivery. Urinary iodide level was measured at delivery to determine the degree of iodine deficiency. Results were matched with a non pregnant control comprising 44 subjects. 5555 birth weights (BW) of term babies in the region were analysed to determine the prevalence of 'small for dates' (SFD) babies in the population. Results show that the level of serum FT4I was elevated very significantly at late gestation (P<.001) but the women were not hyperthyroid, suggesting a marked disturbance in binding of T4 with TBG during pregnancy. Five women with SFD babies were found in 'compensated hypothyroid state' and showed a significant depression (p<.01) in serum T 4/TBG,T4,FT4I and T3 levels to a low normal range, with a concurrent significant rise (p.<.001) in TSH level (above normal range) throughout pregnancy. Incidence of SFD babies was higher (p.<.001) in ID zone (Bassa 15.2%) compared to control zone (Jos 9.8%). It is concluded that a state of maternal 'subclinical (compensated) hypothyroidism' during pregnancy possibly plays an important role in the aetiopathogenesis of SFD babies in Africans. A normal reference range for thyroid parameters at various stages of pregnancy in healthy African women is established for the first time.