Chinese Journal of Physiology

THYROID AND HAIR GROWTH 63

The result given above (VIII) appears to be in contradiction with the undoubted accelerating influence of thyroid in underfed animals. The effect of dosage, however, explains this apparent discrepancy. If figs. 2 and 3 are examined, it will be noted that the hair growth rates in both normal and underfed animals tend to be accelerated with dosages up to 0.6 or 0.7 gm thyroid. In the case of normally fed animals, the rates do not deviate beyond the “normal” range, whereas in underfed animals, the rate with 0.6 gm is definitely outside the range under this condition; it is in fact accelerated. With larger doses than 0.6 gm, the hair growth rate lengthens and with large doses (e.g., 2.2 gm per day) the growth of the hair is very definitely retarded. It is clear, therefore, that 0.6 -0.7 gm constitutes an optimal dosage, and that excess of thyroid brings about retardation instead of acceleration. Since we know that thyroidectomy retards hair growth, and that under-nutrition causes definite regression of the thyroid gland histologically (1, 2), it is clear in the light of our present experiments that the thyroid secretion is fundamentally responsible for the hair growth rate of the individual. In any group of normally fed individuals, we may expect varying levels of thyroid secretion, which may be classified as follows:

(i) Optimal — inducing hair growth at faster rates than the mean (viz., acceleration).

(ii) Slightly sub-optimal

es __ inducing hair growth at rates (iii) Slightly supra-optimal SHo(oves, wats aeacee. (iv) Markedly sub-optimal l inducing hair growth at rates or — beyond the mean (viz., re(v) Markedly supra-optimal ( tardation).

We regard the mean normal rate of hair growth as being just below optimal, and it is obvious that since over-dosage with thyroid depresses hair growth, that sub-optimal rates may be due either to insufficient thyroid secretion or to too much. Not only is the normal distribution of growth rates thus understandable, but also the effect of under-nutrition and thyroid feeding. The former will lower the level of thyroid secretion in all classes and thus lengthen the rate all round. While thyroid feeding will increase the level of thyroid in the body and thus improve (accelerate) the rates of individuals belonging to classes (ii) and (iv), it will depress those belonging to classes (i), (iii) and (v), Thusif the