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Abstract. Serum levels of thyroxine (T₄), triiodothyronine (T₃) and reverse T₃ (rT₃) were studied in dry, early (first trimester) and middle (second trimester) lactating purebred Holstein cows. The study encompassed three different seasons: autumn, winter and spring. Under comfortable weather conditions (temperature 22°C; relative humidity, 40%) or moderately hot (28°C; 60%), January and October, respectively, cows in early lactation exhibited significantly lower levels of T₄ and T₃, and higher values of rT₃ than dry or middle lactating animals. In contrast, during May, when environmental temperature increased (34°C; 40%), a clear-cut shift in T₃/rT₃ratio occurred, and animals in early lactation exhibited the highest T₃ and the lowest rT₃ concentrations.

These findings suggest that in dairy cattle, peripheral thyroid hormone metabolism plays a major role in regulating the homeorhetic responses involved in the maintenance of high priority functions.

Abstract. Milk yields and the circulating profile of T₄, T₃ and rT₃ were assessed during three different seasons of the year, in first trimester lactating (L) and in dry (D) multiparous holstein cows acclimated to distinct weather conditions. Within the thermoneutral zone (18–28°C; 40–60% RH) and regardless of their geographical location, the thyroid hormone profile in all L-cows (n = 50) resembled the so-called euthyroid sick syndrome (T₄, 43.7 ± 7.7 nmol/l; T₃, 1.31 ± 0.10 nmol/l and rT₃, 0.52 ± 0.08 nmol/l). In both groups of animals the T₃/T₄ molar ratio was similar within the entire range of climates encompassed in the study. However, both groups exhibited a significant shift in the T₃/rt₃ molar ratio during cold (10°C; 50%) or hot-dry (34°C; 40%) weather conditions. This shift reaches maximum values (L, 6.5 ± 1.2; D, 7.9 ± 1.0 nmoles/l) under hot-humid conditions (28–42°C; 60–90%). The relative increase of T₃ levels from comfortable to cold or hot environmental temperatures, was significantly higher in L than D animals (30 vs 12%, respectively). Furthermore, only L-cows exhibited a significant decrease in the rT₃/T₄ molar ratio during either type of thermoregulatory demands, as well as a significant increase of T₄ values under heat-acclimation. These results suggest that heat-acclimation in dairy cattle does not depress thyroid gland activity, and lend further support to the notion that adaptive thermoregulatory mechanisms in homeothermic vertebrates, involve adjustments in the peripheral monodeiodinative pathways of thyroid hormones.