Maternal Hypothyroidism in Early and Late Gestation: Effects on Neonatal and Obstetric Outcome

Background: Maternal hypothyroidism may be associated with a variety of adverse neonatal and obstetric outcomes. Whether these outcomes are affected by maternal thyroid status at initial presentation or in late gestation specifically within a dedicated antenatal endocrine clinic remains unclear. The effects of thyroxine dose requirement during pregnancy and serum concentrations of TSH within such clinic settings are still not known.

Objectives: We investigated these outcomes in patients with hypothyroidism during early and late gestation. TSH levels and thyroxine dose requirement during early and late gestation were also evaluated.

Methods: We performed a retrospective study of data from 167 pregnancies managed in the antenatal endocrine clinic. Analysis of outcomes was linked to TSH at first presentation and in the third trimester. Outcome variables included: rate of caesarean section, pre-eclampsia, neonatal unit admission, neonatal weight and gestational age. Controlled TSH was defined as mothers with TSH between 0.1 and 2 with normal free thyroid hormone levels.

Results: The caesarean section (CS) rates were higher in the study cohort (H) compared with the local (C) rate (H = 28.7%, C = 18%). The higher rate in our patient cohort was not due to a higher rate of emergency section nor to a lower threshold for performing elective caesarean section. The infant birthweight (IBW) from mothers with TSH > 5.5 (H1) and mothers with TSH between 0.1 and 5.5 at presentation (H2) was [median (range)] 3.38 (1.73-4.70) vs. 3.45 (1.36-4.76); P = ns. The prevalence of low-birthweight (LBW) infants (< 2.5 g) in groups H1 and H2 was 15% and 4.8%, respectively [odds ratio (OR) = 3.55, 95% confidence interval (95% CI) = 0.96-10.31]. IBW from mothers with TSH > 2 (H3) and mothers with controlled TSH in the third trimester (H4) were similar [3.38 (1.78-4.4) vs. 3.46 (1.36-4.76); P = ns]. The prevalence of LBW in groups H3 and H4 was 9% and 4.9%, respectively (OR = 1.95, 95% CI = 0.52-7.26). The median thyroxine dose (microg) increased significantly during pregnancy (first trimester: 100; second trimester: 125, P < 0.001; and third trimester: 150, P < 0.001) associated with appropriate suppression of TSH levels in the second and third trimesters. Rates of pre-eclampsia or admissions to neonatal units were negligible.

Conclusion: Thyroxine dose requirement increases during pregnancy and thus close monitoring of thyroid function with appropriate adjustment of thyroxine dose to maintain a normal serum TSH level is necessary throughout gestation. Within a joint endocrine-obstetric clinic, maternal hypothyroidism at presentation and in the third trimester may increase the risk of low birthweight and the likelihood for caesarean section. The latter observation was not due to a higher rate of emergency caesarean section nor to a lower threshold for performing elective caesarean section. A larger study with adjustments made for the various confounders is required to confirm this observation.