Thyroid activity in relation to growth rate and body composition in four breeds of sheep.

Four breeds of sheep, Hampshire, Southdown, Cheviot and Scottish Blackface, which differ in mature body size and body composition were intensively reared under an ad libitum or a restricted feeding regime. Thyroid activity was assessed at 13 weeks of age by the 131I uptake method. Slaughter at 24 weeks (ad lib) or 32 weeks (restricted) was followed by carcass dissection. The anticipated differences in body type represented by the 4 breeds was confirmed in terms of size and fatness. The Hampshires had the highest growth rate (liveweight for age) and the lowest thyroid uptake, and the breed differences in thyroid activity were shown to be independent of liveweight. Thyroid activity was positively related to growth rate in the Hampshires and negatively related to growth rate in the 3 other breeds. No association could be found however between the thyroid activity and the proportionate growth of body fat or lean. None of these conclusions were affected by a mild restriction in fed intake.