Thyroid function and autoantibody status among women who spontaneously deliver under 35 weeks of gestation

OBJECTIVE: Recent studies report an association between maternal thyroid auto-antibodies and preterm birth. None have made the important distinction between spontaneous and iatrogenic preterm birth. We investigated the association between spontaneous preterm birth before 35-week gestation and both thyroid function and auto-antibody status. DESIGN: Retrospective nested case-control study, samples retrieved from a biobank of around 8000 samples. PATIENTS: Forty-six cases of spontaneous preterm birth <35-weeks gestation compared with 124 cases who delivered at 40 weeks gestation, matched for maternal age and duration of serum storage. All resulted in a singleton liveborn. MEASUREMENTS: First trimester serum levels of TSH, free T4, free T3, anti-thyroid peroxidase (anti-TPO) and anti-thyroglobulin (anti-Tg) antibodies. RESULTS: Women who had a preterm birth delivered at a median gestational age of 33 (+1 day) weeks gestation (controls 40 + 3 weeks) and had significantly lower birthweight infants [mean (range) weight 1965 (581-3117) grams compared with controls 3526 (2685-4760) grams]. Median levels of anti-thyroglobulin and anti-TPO antibodies were similar in the two groups (Anti-Tg: 3 IU/ml (Range 1-316) v 3 IU/ml (1-691); Anti-TPO 8 IU/ml (3-73) v 9 IU/ml (3-499); P >/= 0.18 for both comparisons, compared using three statistical models). In the preterm group, the median TSH level was 0.70 mIU/l (range 0.04-3.84), similar to controls (0.88 (0.01-2.87), P = 0.1). Free T3 and T4 levels were also similar in both groups. CONCLUSIONS: Thyroid function and thyroid autoantibody status among women who have a spontaneous preterm delivery before 35 weeks gestation are not different from women delivering at term.