PERIPHERAL EFFECTS OF l-THYROXINE ON HAIR GROWTH AND COLORATION IN CATTLE

Abstract

SUMMARY An experiment was conducted to test the hypothesis that thyroid secretion may have a direct effect on hair growth and coloration in cattle, offering a common basis both for photoperiodic and thermally induced changes in the coat. 600 μg l-thyroxine were applied daily over an area of 100 cm2 of the left lumbar region of six cows for 46 days, and an identical amount of solvent to the corresponding area of the right lumbar region which served as a control. The cows were subjected to a constant day length of 12 hr, a constant air temperature (31·7±0·8° C) and constant relative humidity (69·0 ± 5·6%). Hair growth was determined by the weight of hair grown on the 100 cm2 areas during the experimental period after an initial clipping. The amount of pigment in 100 mg hair was determined spectrophotometrically following digestion of hair in 0·5 n-NaOH. The topical application of l-thyroxine increased hair growth by 18% and pigmentation by 18·7% above the values on the control areas. This suggests a possible role of the thyroid in the control of the annual cycle in hair growth and shedding in cattle.