Total thyroxine (T(4)) concentrations were below the non-breed-specific reference range in 42 of 46 healthy young greyhounds (91.3 per cent) and 16 (34.8 per cent) were at or below the limit of detection of the assay. Free T(4) concentrations were below the standard reference interval in 20.5 per cent of the animals and 13 per cent were at or below the limit of detection of the assay. In contrast, all the dogs' total tri-iodothyronine concentrations were within or above the non-breed-specific reference range and 67 per cent were within the upper half. All the dogs' thyroid stimulating hormone concentrations were within the non-breed-specific reference range. The results show that young greyhounds have markedly lower total and free T(4) concentrations than other breeds, and neither analyte can reliably be used to investigate the hypothyroidism in this breed as values were found below the limit of detection of each respective assay.