Thyroid hormone and hair growth

Diffuse loss of scalp, body and eyebrow hair are well‐recognized clinical signs of thyroid deficiency, known to generations of medical students. Thyroid hormone also affects hair growth in other species. In rats thyroxine increases and thiouracil decreases the final length of hair (reviewed 1). Thyroxine increases the rate of hair growth in sheep 2 and, in badgers, hair growth and seasonal moulting ceased following thyroidectomy and resumed when the animals were treated with thyroxine. 3 Objective studies of the effect of thyroid hormone on human hair growth are, however, thin on the ground. Freinkel and Freinkel found high telogen counts in scalp hair in nine hypothyroid subjects, which were corrected following thyroid replacement therapy. 4 Jackson and colleagues compared scalp hair diameters in groups of hypothyroid women, women with female pattern balding and normal women. 5 In women with normal hair density, hair diameters showed a symmetrical distribution with a single peak at 80 µm. Mean hair diameter was reduced in the hypothyroid and euthyroid balding women and in both groups hair diameters showed a similar bimodal distribution. They suggested that hair loss in hypothyroidism only occurs when the condition adds to or accelerates a familial tendency. The effect of thyroid status on human hair growth has also been demonstrated at the cellular level. Using DNA flow cytometry, Schell et al. observed that cell proliferation indices were reduced in hair bulbs of hypothyroid subjects and increased in hyperthyroidism compared with normal values. 6