Far and away the most common causes of hair loss are hypothyroidism, low ferritin, and high androgen levels (such as testosterone, androstenedione, and DHEA).
Less common but still prevalent are the apparently autoimmune condition alopecia areata, and the stress-induced telogen effluvium.
But one I also see quite a bit is biotoxin-related hair loss, due to elevation in the biotoxin marker TGFb1.
Biotoxins and Hair Loss
Toxic mold exposure can cause hair loss, and it generally elevates the inflammatory cytokine TGFb1. According to this study, TGFb1 has a negative regulating effect on hair follicles; in other words, as TGFb1 goes up, hair growth goes down.
According to this study, this relationship might be due to the fact that TGFb1 tends to lower DHEA.
DHEA and Hair Loss
DHEA (Dehydroepiandrosterone) is one of three major cholesterol-based hormones produced by your adrenal glands (the other two being aldosterone, and cortisol.) DHEA is a precursor hormone for estrogen and testosterone in both men and women. Too much of it can cause androgenic alopecia, as mentioned above—but too little of it can cause hair loss also. As we age, DHEA lowers naturally; in our 70s we only produce about 5% as much DHEA as we produced at twenty. (This might have something to do with the fact that hair loss increases with advancing age.)
The mechanism for how low DHEA induces hair loss is not entirely clear. In autoimmunity, DHEA tends to be low (while cortisol tends to be high, at least initially, in order to help the body deal with the inflammation—and DHEA and cortisol are typically opposed to one another.) Since alopecia areata is considered autoimmune, low DHEA might be part of the mechanism for this type of hair loss.
But this study implies that the mechanism has more to do with fibrosis: that is, low DHEA (secondary to high TGFb1) may actually encourage fibrotic (scarring) hair loss.
If you’ve already tried the usual tests (listed here) to diagnose hair loss to no avail, consider the possibility of too low, rather than too high, DHEA, and/or of high TGFb1 (higher than 2380, that is; the upper limit of the reference range keeps going up).
If DHEA is low, you can consider supplementation. If you find the latter is elevated as well, I’d certainly recommend getting a workup to identify the cause.