Chapter 14 463 ages 13–72. The data was separated into three groups: those with diffuse thinning (n = 40), those with hypothyroidism (n = 14), and a “normal” group (n = 71). For each group the data was analyzed by linear regression analysis, despite the fact that graphs of the data appeared to display curvature with the highest data points in the 30s and 40s, and the data for the hypothyroid and normal group failed to reach significance. Therefore, no conclusion can be drawn with respect to the peak age for maximum diameter from this study. The first four smaller studies17-20 are in reasonable agreement with the conclusions from the two larger ones, indicating that the age for maximum hair diameter for females is near the age of 40. One exception is the study by Birch et al.23 on more than 300 Caucasian females, providing the conclusion that the age for maximum diameter was ~ 30. Mirmirani and Dawson et al.24 have shown that post-menopausal women have significantly lower hair fiber diameters, lower frontal scalp hair density, and lower growth rates than pre-menopausal women. Optical fiber diameter and phototrichogram methods were used to quantitate these hair parameters. Two studies were conducted by these scientists. An initial study included 44 women, 20 in the post-menopausal group and 24 in the pre-menopausal group. The second study included 177 women (ages 40–60) with 54 in the pre-menopausal, 33 in a peri-menopausal group (irregular periods or cessation of periods for less than 12 months), and 90 in the post-menopausal group. Average fiber diameters were significantly higher in pre-menopausal versus post-menopausal women in the frontal site, but not in the occipital site. The data also suggested that this fiber diameter effect was independent of age. In the expanded study by Mirmirani and Dawson, average fiber diameters on the frontal site were significantly higher in pre- menopausal versus post-menopausal and peri-menopausal women. However, there was no significant difference in hair fiber diameters in peri-menopausal and post-menopausal women. In the Robbins et al. study,16 the authors measured mean hair fiber diameters on 250–400 hairs from each of left and right