Effects of complex organohalogen contaminant mixtures on thyroid homeostasis in hooded seal (Cystophora cristata) mother–pup pairs

• Organohalogen contaminants in nursing hooded seal mothers and pups were reported.
• Multivariate associations between thyroid hormones and contaminants were shown.
• Specific contaminants appear to affect thyroid homeostasis in mothers and pups.
• Similar thyroid responses may reflect linked mother–pup exposure or thyroid effects.
• Some contaminants may have higher thyroid disruptive potency in pups (e.g. OH-PCBs).

Many lipid-soluble and phenolic compounds present in the complex mixture of orgaohalogen contaminants (OHCs) that arctic wildlife is exposed to have the ability to interfere with the thyroid hormone (TH) system. The aim of this study was to identify compounds that might interfere with thyroid homeostasis in 14 nursing hooded seal (Cystophora cristata) mothers and their pups (1–4 d old) sampled in the West Ice in March 2008. Multivariate modelling was used to assess the potential effects of measured plasma levels of OHCs on circulating TH levels of the measured free (F) and total (T) levels of triidothyrine (T3) and thyroxine (T4). Biological factors were important in all models (e.g. age and sex). In both mothers and pups, TT3:FT3 ratios were associated with α- and β-hexachlorocyclohexane (HCH), ortho-PCBs, chlordanes and DDTs. The similarities between the modelled TT3:FT3 responses to OHC levels in hooded seal mothers and pups most probably reflects similar exposure patterns, but could also indicate interconnected TH responses. There were some differences in the modelled TH responses of mothers and pups. Most importantly, the negative relationships between many OH-PCBs (particularly 3’-OH-CB138) and TT3:FT3 ratio and the positive relationships between TT4:FT4 ratios and polybrominated diphenyl ether [PBDE]-99, -100 and 4-OH-CB107 in pups, which was not found in mothers. Although statistical associations are not evidence per se of biological cause–effect relationships, the results suggest that thyroid homeostasis is affected in hooded seals, and that the inclusion of the fullest possible OHC mixture is important when assessing TH related effects in wildlife.