Thyroxine, triiodothyronine and iodide in different breeds of newborn calves.

Plasma thyroxine (T4), triiodothyronine (T3) and iodide levels were measured from birth until 30 days of age in 17 Holstein x Friesian (HF), 7 Salers and 7 Charolais calves born at term. The response of plasma T4 and T3 levels to bovine thyrotropin (TSH) injection was also compared in two groups of 9 HF calves 3hrs and 21 days, respectively, after birth. In the HF calves, plasma T4 and T3 levels increased from birth to 6 hrs (when the rectal temperature of these calves decreased slightly), then diminished until day 7 and remained stable until 30 days of age. In Salers calves, changes in the plasma T4 and T3 levels were not different from those observed in HF animals. However, in Charolais calves, there was no significant increase in plasma T4 and T3 levels after birth. Twelve and 24 hrs after birth, the plasma iodothyronine levels measured in these animals were lower than in HF calves. In Salers and Charolais calves a positive linear relationship was demonstrated between plasma T4 (or T3) levels at birth and the birth weight. In the three groups of calves, plasma iodide levels decreased from birth until 10 days of age, then remained stable until the end of the first neonatal month. Intravenous injection of the same dose of bovine TSH (5 mU/kg of body weight) induced a rise in the plasma T4 and T3 levels which was significantly more intense in 21-day old calves than in 3-hr old calves.