Several skin conditions, including psoriasis, eczema, and seborrheic dermatitis, don't directly lead to hair loss, but cause inflammation (a symptom of these conditions) near the brow that may be to blame. The urge to scratch and rub the inflamed area can indirectly cause eyebrow hairs to fall out. Loss that occurs in this manner is usually patchy, but as it is generally non-scarring (the hair follicle is intact), eyebrows can and often do grow back.
The WHI was a very large, very prominent long-term study which looked at the health effects of HRT. While the study did find a few benefits (women who used HRT had fewer hip or bone fractures and were less likely to develop colorectal cancer), it found a number of risks, including an increased chance of heart attack, stroke, cancer, and blood clots.
Your body has an amazing capacity to send out signals when it’s in trouble, and your eyebrows are no exception. Aging causes harmless thinning of the hair, in general, so it may contribute to less-than-lush eyebrows. But when the thinning is accompanied by other symptoms, a visit to your doctor or dermatologist is in immediate order. With timely treatment, your brows—and, more important, your body—can make a steady and healthful recovery.
The real culprit appears to be dihydrotestosterone (DHT), a more potent form of testosterone. DHT is made from testosterone by a specific enzyme in the body, and while both testosterone and DHT are known to have a weakening effect on hair follicles, there appears to be something unique about the conversion process of testosterone to DHT that relates to thinning hair. This is why some drugs that are marketed for hair loss block the conversion of testosterone to DHT. (It’s important to note, however, that these drugs tend to be less effective in women than men, and that one of them—finasteride—is only approved for hormonal hair loss in men, not women. What’s more, the drug has been associated with increased risk of sexual side effects, depression, nausea, hot flashes, and increased estrogen levels—and too much estrogen is its own risk factor for thinning hair; more on that below.)
Thyroid hormone receptors were detected in both dermal and epithelial compartments of the human pilosebaceous unit. T4 and T3 decrease the apoptosis of hair follicles and T4 prolongs the duration of anagen in vitro. Thyroidectomy delays initiation of anagen. Administration of thyroxine advances anagen, initiation of which is however delayed once toxic doses are given. Therefore, ratio of telogen to anagen hairs is increased in hypothyroidism as well as hyperthyroidism. Thus, the hair follicles are affected in thyroid disorders, and madarosis is caused due to disturbances in hair cell kinetics. Hypothyroidism is associated with generalized hair loss probably due to coarse, dull, and brittle hair with reduced diameter. The eyebrows and eyelashes may also be lost. Loss of lateral one-third of eyebrows known as Hertoghe sign is a characteristic sign of hypothyroidism. Some people also refer to it as Queen Anne's sign, after Anne of Denmark whose portrait with shortened eyebrows has been interpreted by some as indicative of the presence of goiter, even though such a fact has not been proved by any known sources of information. Madarosis may even be the presenting sign in hyperthyroidism. In hyperthyroidism, there is thinning with breaking off and shortening of hair. Madarosis can also occur in hypopituitarism, hypoparathyroidism, and hyperparathyroidism.