The relative proportions of telogen (club) to anagen (growing) hairs in the scalp were examined in hypothyroid subjects with hair loss to asses the basis for the alopecia which occurs when thyroid hormone is lacking. Patients with spontaneous as well as iatrogenic myxedema were examined and serial measurements were secured during restoration of eumetabolism. In all instances, deficiency of thyroid hormone was associated with an increase in the percentage of telogen hairs. Normal telogen-anagen hair relationships were restored during replacement therapy with thyroid hormone; this response was elicited even during the concurrent administration of antithyroid drugs (ie, thiourylenes). The response to treatment with thyroid hormone and the temporal correlations indicate that the alopecia is mediated via effects of the hormone on the initiation as well as the duration of hair growth.
Freinkel RK, Freinkel N. Hair Growth and Alopecia in Hypothyroidism. Arch Dermatol. 1972;106(3):349-352. doi:10.1001/archderm.1972.01620120037007