Changes in grey seal (Halichoerus grypus) serum thyroid hormones with age, sex and during lactation were investigated. Concentrations were related to polychlorinated biphenyl (PCB) uptake and exposure in pups and females. There was no effect of sex or stage of lactation on either free or total thyroxine (T4) and triiodothyronine (T3), but significant age-related variability was seen. Pups had significantly higher concentrations of total T4 than adults (p <0.0001) and serum levels declined with age. In general, there was no significant relationship between PCB exposure to pups through female milk or between PCB blubber concentrations and thyroid hormone levels, when the effect of age was controlled for. However, the ratio T3:T4 was significantly correlated with the concentration of one congener, CB169 (p = 0.048), when stage of lactation was used as a covariate. In conclusion, serum thyroid hormone levels may be less useful biomarkers of contaminant exposure in seals than other biochemical measures. Although concentrations vary predictably with age, other factors, such as condition and nutritional status, may compromise their usefulness in assessing contaminant-induced hormone disruption in pinnipeds.