Background Female pattern hair loss is common but estimates of its prevalence have varied widely. The relationships between the clinical diagnosis of female pattern hair loss and objective measurements of hair density and hair diameter have not previously been evaluated.
Objectives To determine the prevalence of female pattern hair loss and to relate the clinical findings to hair density and hair diameter.
Methods We examined 377 women, aged 18–99 years, who presented to a general dermatology clinic with complaints unrelated to hair growth (the unselected sample). A second group of 47 women referred with typical female pattern hair loss was included in analyses of the relationships between hair density, hair diameter and the clinical diagnosis. Hair density was measured using a photographic method. In each subject the major and minor axis diameters were measured in a random sample of 50 hairs.
Results Six per cent of women aged under 50 years were diagnosed as having female pattern hair loss, increasing to 38% in subjects aged 70 years and over. The mean ± SEM hair density was 293 ± 61·3 hairs cm−2 at age 35 years, falling to 211 ± 55·1 hairs cm−2 at age 70 years. Hair density showed a normal distribution in the unselected sample. Most women classified as having female pattern hair loss had hair densities within the lower half of the normal distribution. The perception of hair loss was determined mainly by low hair density (anovaP < 0·001), but there was overlap in hair density between women classified as having Ludwig I hair loss and the no hair loss group, which was partly accounted for by differences in mean hair diameter (anovaP < 0·001). Low hair density was associated with fewer hairs of all diameters.
Conclusions Hair density in women is distributed as a normal variable, indicating that it is determined as a multifactorial trait. Women with female pattern hair loss have a hair density which falls below the mean but lies within the spectrum of the normal distribution, although other factors, including hair diameter, may affect the subjective impression of hair loss. The hair diameter data suggest that low hair density is not due to progressive diminution in hair follicle size and that follicular miniaturization may occur within the space of a single hair cycle.