Hysterosalpingography (HSG) using oil-soluble contrast medium (OSCM) improves pregnancy rates1, but results in severe and persistent iodine excess2,3 potentially impacting the fetus and neonate. The aim of this study was to determine the incidence of thyroid dysfunction in newborns conceived within six months of OSCM HSG.
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) study4 that assessed iodine excess and thyroid dysfunction of 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 thyroid stimulating hormone (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 subclinical hypothyroidism in the newborn.
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 (95%CI -0.5, -0.1; p= 0.029) despite lower FT4 levels (95% CI -2.1, -0.1 pmol/L; p=0.041).
In conclusion, pre-conceptional OSCM HSG in women did not increase neonatal subclinical hypothyroidism or congenital hypothyroidism. However, gestational iodine excess was associated with a paradoxical lowering of neonatal FT4 levels despite lower TSH levels. We speculate these changes reflect iodine excess on the fetal hypothalamic-pituitary axis.