Plasma Thyroid Hormones and Cholesterol in the Newborn of Genetically Different Types of Cattle in a Tropical Environment

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Abstract
The patterns of plasma concentrations of thyroxine (T4), triiodothyronine (T3) and cholesterol were determined in tropically adapted (ZX) and temperate (SH) breeds of cattle in the first 6 days of their lives in a tropical environment. T4 concentrations were high at birth and fell to within normal adult values at 6 days of age. In contrast, plasma cholesterol levels were low at birth and increased daily to the highest values on day 6. T3 levels increased from birth to maximum values at 12 h after birth and then declined progressively. The T4/T3 molar ratios were high at birth and decreased to lower values on day 2. T4 and cholesterol levels were higher (p < 0.001) in ZX than in SH calves, and in calves born in winter were higher (p < 0.001) in females than in males. There were significant (p < 0.001) animal within-breed differences in all parameters. The results are discussed with reference to the use of physiological and biochemical indices for the early selection of cattle suited to tropical conditions.

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