Thyroid Function Testing in Pregnancy

Introduction: Thyroid physiology changes during pregnancy due to the structural similarity between thyroid stimulating hormone (TSH) and beta human chorionic gonadotropin (HCG) produced by the placenta. Clinical guidelines from both the American Thyroid Association (ATA) and the American College of Obstetrician and Gynaecologists (ACOG) recommend against universal screening for thyroid disorder in pregnancy. Rather they favour, targeted case finding before pregnancy. Another area of controversary is the reference ranges used to identify thyroid dysfunction in pregnancy. First line guidance is for maternity units to develop their own references ranges based on women reflective of their local population. But very few units have carried out this reference range development. The aim of this thesis was to improve our understanding of thyroid function and dysfunction in pregnancy so as to better serve our patients. Firstly, we carried out prospective thyroid function tests (TFT) in healthy women with no history of thyroid disorder across their gestations. We wanted to evaluate if longitudinal, rather than cross sectional, TFT across pregnancy were more accurate in determining associations between thyroid function and differing obstetrical, delivery and neonatal outcomes. We, secondly, wanted to see how many cases of overt hypothyroidism, subclinical hypothyroidism (SCH), overt hyperthyroidism and subclinical or transient hyperthyroidism were identified in our population of women who would not have met the criteria for the traditional targeted case finding approach. Thirdly, we wanted to perform a qualitative study of women’s attitudes and responses to being diagnosed with thyroid disorder during pregnancy. Materials and Methods: We recruited 699 women over the age of 18 whom were deemed able to give informed consent and whom had no history of thyroid disorder. We performed longitudinal TFTs in these women across the three trimesters of their pregnancies. We also measured their thyroid peroxidase antibody (TPOAb) levels during the pregnancy. 6 women who were found to have abnormal TFTs on their initial first trimester testing in the longitudinal study were then interviewed for the qualitative study. This interview was performed by a researcher assistant whom had no clinical relationship with the women. Results: 1) We divided our participants into three longitudinal trajectories based on changes in their TSH levels across their gestation. We found that one trajectory had, statistically significant, higher rates of TPOAb positivity and spontaneous vaginal delivery (SVD). We also found another one of the trajectories had statistically significant higher rates of multiple gestation. 2) We did not find that TPOAb positivity in itself incurred a higher rate of any obstetric, delivery or neonatal complications that we evaluated. 3) We found a rate of overt hyperthyroidism of 1.2% in our study population. 4) We found a rate of overt hypothyroidism of 5% in our study population. 5) We identified themes and subthemes relating to women’s attitudes towards receiving a diagnosis of thyroid abnormality during their pregnancy. Conclusion: We showed that our population could be divided into previously identified TFT trajectories based on changes in TSH level during the participants gestation. We found some of these trajectories had statistically significant differences in SVD, multiple gestation and TPOAb positivity. We did not find that TPOAb positivity in and of itself increased the rates of any of the obstetric, delivery and neonatal outcomes studied.