Svalbard's arctic foxes experience large seasonal variations in light, temperature and food supply throughout the year, which may result in periods of starvation. The aim of this work is to investigate if there are seasonal variations in post-absorptive plasma thyroid hormones (free thyroxin (fT 4 ), free triiodothyronine (fT 3 ) and reverse triiodothyronine (rT 3 )) and metabolites (free fatty-acids (FFA) and b-hydroxybutyrate (b-OHB)) with season and their response to starvation and re-feeding. The concentrations of post-absorptive free triiodothyronine were significantly higher in November than May, while those of thyroxin, reverse triiodothyronine, free fatty-acids and b-hydroxybutyrate remained unchanged. Possible explanations for the seasonal variations in free triiodothyronine are discussed. There were no significant changes from post-absorptive concentrations of thyroxin and reverse triiodothyronine in starved and re-fed foxes. However, free triiodothyronine concentrations decreased during starvation and increased again with re-feeding both in May and November. Starvation induced high levels of free fatty acids in both May and November, indicating increased lipolysis. There was a significant increase in b-hydroxybutyrate in November only, indicating that arctic foxes are capable of protein conservation during starvation.