93 Safety of HSG with oil-based contrast medium 5 may vary between Japanese and Caucasian women, possibly because of a different background risk (i.e. iodine-rich diet). The consumption of iodine-rich foods by mothers in Japan has been shown to lead to neonatal hypothyroidism (134). This may be reflected in the overall risk for congenital hypothyroidism, which is 0.7% in Japan compared with 0.04% in the Netherlands (135, 136). Data on Asian women suggest that neonatal thyroid dysfunction after HSG is related to the amount of oil-based contrast used during the procedure, although volume of contrast was not reported for all procedures (94). It is unclear whether Caucasian women with an underlying thyroid disease are also at risk of developing hypothyroidism after an HSG with oil-based contrast. Until further studies have been performed, it is suggested that women with overt thyroid disease should not receive an HSG with oil-based contrast. In current practice, routine thyroid screening for women with subfertility varies. According to the NICE guidelines thyroid screening is not recommended as routine measurement in asymptomatic women presenting with subfertility (1). However, the ACOG committee opinion on fertility work-up does recommend routine thyroid testing for all subfertile women (137). Moreover, the 2017 American Thyroid Association guidelines for the diagnosis and management of thyroid disease during pregnancy and the post-partum period advises maintaining serum TSH concentrations below 2.5 mIU/l preconceptually in the subfertility setting (138). In this systematic review of complications of HSG from 1928 onwards, the most frequently reported complication with oil-based contrast is intravasation, occurring in 2.7%. Only four cases of serious consequences of oil embolisms in subfertile women have been published since 1928. Therefore, safety concerns should not be the reason to deny the use of oil-based contrast for tubal testing in women with unexplained subfertility. Further studies on the effect of oil-based contrast on maternal and neonatal thyroid function in Caucasian women are suggested. Furthermore, future research should investigate the mechanism of the pregnancy-enhancing effect of oil-based contrast. By gaining knowledge on the mechanism of action, it would be possible to determine which women would benefit most from an HSG with the use of oil-based contrast.
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