Thyroid function in normal pregnancy

A study to establish gestation-specific reference intervals for thyroid function tests in normal singleton pregnancy

OBJECTIVE

To establish gestation-specific reference intervals for thyroid function tests in normal singleton pregnancy.

STUDY DESIGN

Cross-sectional observational study performed in the Obstetric and Gynaecology department, West Middlesex University Hospital. A single blood sample from 335 pregnant women at various gestations of pregnancy was analysed for thyroid function. FT4, fT3, TSH values at each gestation of pregnancy were calculated.

RESULTS

From the fT3, fT4 and TSH results, a 95% reference interval was calculated for each hormone for each week of pregnancy.

CONCLUSION

We calculated gestation-specific reference intervals for thyroid function tests throughout pregnancy to facilitate clinical management of thyroid disease in pregnancy.

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 T(4), T(3), TSH, TBG, thyroxine binding capacity (TBK), free thyroxine index (FT(4)I=T(4)/TBK) and T(4)/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 FT(4)I was elevated very significantly at late gestation (P<.001) but the women were not hyperthyroid, suggesting a marked disturbance in binding of T(4) 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, T(4), FT(4)I and T(3) 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.

The effects of iodoprophylaxis on thyroid size during pregnancy

Thyroid gland size was ultrasonographically determined in 35 pregnant women who live in an area with moderate iodine deficiency. Iodide salt was administered to group A (n = 17), whereas group B (n = 18) was used as a control. Each group was tested for thyroid-stimulating hormone serum levels, iodine excretion, and thyroid volume. In both groups thyroid-stimulating hormone levels were similar and did not change throughout pregnancy. The iodine excretion at the third trimester in the treated group was significantly (p less than 0.01) higher than that of group B (100.0 +/- 39.0 versus 50.0 +/- 37.0 micrograms iodine per 24 hours, respectively). Initially, thyroid volume did not differ between the two groups. At the end of pregnancy, no difference was found in thyroid size in group A, whereas in the untreated women it increased significantly (p less than 0.0001) with a mean increase of 1.6 +/- 0.6 ml (16.2% +/- 6.0%). These results show that the increased thyroid size in the control group was mainly a result of relative iodine deficiency and that iodoprophylaxis should be warranted even in areas with moderate iodine deficiency to prevent the increase in thyroid size and, probably, to avoid the risk of maternal and fetal hypothyroidism.