IMPE Abstracts

Background: Hysterosalpingography (HSG) using oil-soluble contrast medium (OSCM) improves fertility with pregnancy rates of approximately 40% reported in women within six months of the procedure. However, OSCM has high iodine content and long half-life resulting in severe and persistent maternal iodine excess for up to 6 months. Any fetus conceived during this time would be exposed to a high iodine load with the potential for thyroid dysfunction from the Wolff- Chaikoff effect, as described with maternal exposure to high dietary iodine or drugs such as amiodarone.

Aim: The aim of this study was to determine the incidence of congenital hypothyroidism and transient elevated levels of thyroid stimulating hormone (TSH) in newborns conceived within six months of an OSCM HSG.

Methods: A prospective newborn study was undertaken in Auckland region, New Zealand from 2020-2022. The cohort consisted of 57 newborns conceived within 6 months to a group of women who underwent OSCM HSG as part of SELFI (Safety and Efficacy of Lipiodol in Fertility Investigations) study, that assessed iodine excess and thyroid dysfunction in women post-HSG. All newborns had a dried blood spot card for TSH measurement after 48 hours of birth (newborn screening). 41 neonates also had a heel prick serum sample at one week for TSH, free thyroxine (FT4) and free triiodothyronine (FT3). Maternal urine iodine concentration (UIC) and TSH in the six months after OSCM HSG were retrieved from the SELFI study. The primary outcome was the incidence of congenital hypothyroidism or transient TSH elevation in the newborn.

Results: There was no evidence of primary hypothyroidism on newborn screening (whole blood TSH 2-10mIU/L). All neonates tested at one week had serum TSH, FT4 and FT3 in the age-appropriate reference range. However, increasing maternal peak UIC levels were associated with lower TSH levels (95%CI -0.5, -0.1; P= 0.029) despite lower FT4 levels (95% CI -2.1, -0.1 pmol/L; P=0.041).

Conclusion: Pre-conceptional OSCM HSG in women did not increase congenital hypothyroidism or neonatal transient TSH elevation as expected from Wolff- Chaikoff effect. However, early gestational iodine excess was associated with a paradoxical lowering of neonatal FT4 levels despite lower TSH levels. We speculate these changes reflect the effect of iodine excess on the fetal hypothalamic-pituitary axis possibly by influencing deiodinase activity in the brain. Further human and animal research are warranted to study the effects of early, severe and prolonged iodine excess on fetal thyroid and brain.