Evaluating thyroid hormone disruption: Investigations of long-term neurodevelopmental effects in rats after perinatal exposure to perfluorohexane sulfonate (PFHxS)

Thyroid hormones are critical for mammalian brain development. Thus, chemicals that can affect thyroid hormone signaling during pregnancy are of great concern. Perfluorohexane sulfonate (PFHxS) is a widespread environmental contaminant found in human serum, breastmilk, and other tissues, capable of lowering serum thyroxine (T4) in rats. Here, we investigated its effects on the thyroid system and neurodevelopment following maternal exposure from early gestation through lactation (0.05, 5 or 25 mg/kg/day PFHxS), alone or in combination with a mixture of 12 environmentally relevant endocrine disrupting compounds (EDmix). PFHxS lowered thyroid hormone levels in both dams and offspring in a dose-dependent manner, but did not change TSH levels, weight, histology, or expression of marker genes of the thyroid gland. No evidence of thyroid hormone-mediated neurobehavioral disruption in offspring was observed. Since human brain development appear very sensitive to low T4 levels, we maintain that PFHxS is of potential concern to human health. It is our view that current rodent models are not sufficiently sensitive to detect adverse neurodevelopmental effects of maternal and perinatal hypothyroxinemia and that we need to develop more sensitive brain-based markers or measurable metrics of thyroid hormone-dependent perturbations in brain development.

Impact/Purpose

Background/Overview: Perflourinated chemicals (PFAS) represent a ubiquitous group of environmental contaminant and have been shown to disrupt the thyroid axis. Thyroid hormones (TH) are critical for brain development. Serum measures of TH are used for regulatory decision-making purposes because of concern for neurodevelopmental disorders, serum T4 representing a key integrator node in the Adverse Outcome Pathway (AOP) upstream from impaired neurodevelopment. Predictions stemming from altered T4 in this AOP are largely based on data from agents known to disrupt the thyroid system by inhibiting TH synthesis at the level of the thyroid gland (i.e., propylthiouracil, methimazole). Many other chemicals with distinct modes of action are also capable of reducing serum T4, but confirmation of their link to neurodevelopmental deficits has remained elusive. One of the proposed modes-of-action for PFAS chemicals is through their affinity for serum binding proteins. This study was performed to assess the thyroid disruption and potential neurotoxicity associated with developmental exposure to one member of the PFAS family, perfluorohexane sulfonate (PFHxS). Relevancy to EPA Program/Regional Research Needs/Priorities: This work addresses Chemical Safety for Sustainability (CSS) Adverse Outcome Pathway Discovery and Development (AOPDD 17.01), Thyroid Related AOPs and is relevant to OPPTS and OW needs for decision making. Name(s) of Program Office Reviewer(s) of Earlier Drafts: Chris Lau, NHEERL; Michele Taylor, NCEA Program Office/Regional Office Co-Authors: None Study Description: Pregnant rats were exposed to PFHxS from gestational day 6 (GD6) until postnatal day 21 (PN21). PFHxS was evaluated alone (3 dose levels plus control) and together with a mixture of 12 environmentally relevant endocrine disruptors comprised of phthalates, pesticides, UV filters, bisphenol A and butyl paraben. Dam and pup serum were collected for TH analysis and thyroid gland gene expression. Gene expression in the brain was examined in the cortex sampled from offspring on PN17 for candidate genes previously shown to be sensitive to neonatal T4 reductions. Offspring were evaluated on a series of neurobehavioral tests to evaluate developmental neurotoxicity. Major Observations and Results: Exposure to PFHxS lowered T4 and T3 levels in both dams and their offspring, but without any apparent activation of the hypothalamic-pituitary-thyroid axis, as no significant changes to TSH levels, thyroid gland weight, histology, or gene expression were observed. The thyroid hormone deficiency did not cause changes to behavior, such as impaired learning or increased activity levels. Rather, we observed only slight effects on offspring’s learning and memory that was not correlated to T4 levels, but altered activity levels that was suggestive of disrupted sexual brain differentiation. Impact/Potential Implications of the Findings: These data suggest that despite reductions in serum T4 in dams and offspring, according to the metrics examined here, the developing brain was not adversely affected by maternal exposure to PFHxS. Thus, standard behavioral assays appear insensitive to adverse developmental effects of hypothyroxinemia/hypothyroidism, so that more sensitive and reliable brain-based markers of thyroid hormone-dependent neurotoxic insults are needed to protect humans against disrupted thyroid function associated with PFAS exposure. Findings Advancing Existing Scientific Knowledge: These findings indicate that more sensitive metrics of TH-mediated neurodevelopmental impairment are needed for the further development and refinement of quantitative AOPs for thyroid disruption. In their absence, the use of serum TH appears to be protective of the fetus and developing neonate.

Citation

Ramhoj, L., U. Haas, M. Gilbert, C. Wood, T. Svingen, D. Usai, A. Vinggaard, K. Mandrup, AND M. Axelstad. Evaluating thyroid hormone disruption: Investigations of long-term neurodevelopmental effects in rats after perinatal exposure to perfluorohexane sulfonate (PFHxS). Nature Publishing Group, London, UK, 10(1):2672, (2020). [DOI: 10.1038/s41598-020-59354-z]

DOI: Evaluating thyroid hormone disruption: Investigations of long-term neurodevelopmental effects in rats after perinatal exposure to perfluorohexane sulfonate (PFHxS)