This is an autoimmune disorder that is also known as “spot baldness” and can result in hair loss ranging from just one location (alopecia areata monolocularis) to the entire scalp (alopecia totalis), or to every hair on the entire body (Alopecia areata universalis). Although it is thought to be caused by hair follicles becoming dormant, what triggers alopecia areata is not exactly known. In most cases, the condition corrects itself.

Reproductive hormone changes can also play a part, and dermatologists believe the same factors that cause hair loss from the scalp, a common problem for women as they age, may contribute to thinning brows. Nanette Santoro, M.D., ob/gyn and professor of reproductive endocrinology at the University of Colorado at Denver, says, “Abrupt hormone changes can cause sudden hair loss (telogen effluvium) that recovers over about six months' time. It happens postpartum to many women and can happen at menopause.”
Both of these hairy issues are related to excess androgens that are part of PCOS.  Our hair follicles respond overzealously to testosterone creating both hair loss and hair growth. This is because the follicles on the face tend to stretch and grow causing the hair to get stuck and not fall out when they should. Conversely, the scalp hair follicles actually shrink in response to androgens and the hair can fall out or just not be as robust.

Thyroid hormone receptors were detected in both dermal and epithelial compartments of the human pilosebaceous unit.[48] T4 and T3 decrease the apoptosis of hair follicles and T4 prolongs the duration of anagen in vitro.[49] 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.[50] 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.[51] The eyebrows and eyelashes may also be lost. Loss of lateral one-third of eyebrows known as Hertoghe sign[38] is a characteristic sign of hypothyroidism.[52] Some people also refer to it as Queen Anne's sign,[53] 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.[21] In hyperthyroidism, there is thinning with breaking off and shortening of hair.[54] Madarosis can also occur in hypopituitarism, hypoparathyroidism,[21] and hyperparathyroidism.[55]

But, interestingly — and what most people don't initially realize — is that eyebrow hair loss is often a sign of an internal disease. The classic example of this is an under-active or over-active thyroid. Loss of the outer third of the eyebrow, in particular, is a characteristic sign of an under-active thyroid. Fortunately, thyroid conditions are easily treatable, usually with oral medications, but the medications generally have to be taken for life. The eyebrows should grow back, but there is always the possibility that they may not. The thyroid gland plays a huge role in our bodies, so it is important to have any thyroid issues treated.

Scurf refers to the scales and greasy crusts that accumulate along the hair shaft and indicates the presence of seborrhea.[26,27] Collarettes are composed of hard fibrinous scales[25,28] surrounding each individual eyelash. They travel upward along with the growth of the lashes and are indicative of staphylococcal infection. Sleeves or cylindrical dandruff comprise scales that form a cuff around the lash root and are connected with it, in contrast to greasy scales which are not connected to the lash root.[29] Sleeves indicate infestation with Demodex folliculorum.
In extreme cases, such as disordered eating and anemia, an iron deficiency may be to blame for thinning brows. “Even if you don’t have anemia, and you have low levels of stored iron, that could contribute to hair loss,” says Rajani Katta, M.D., a dermatologist and clinical assistant professor of medicine at Baylor College of Medicine, who studies the link between nutrition and hair loss. Iron is found in meat, fish, and other animal products, plus beans and legumes, so vegans and vegetarians might be more likely to be low in iron. Your derm can do a ferritin blood test to check your iron levels. But don’t start an iron supplement without medical recommendation. Too much iron can also have negative effects, says Dr. Katta.
Hormones are cyclical. Testosterone levels in some men drop by 10 percent each decade after thirty. Women's hormone levels decline as menopause approaches and drop sharply during menopause and beyond. The cyclic nature of both our hair and hormones is one reason hair loss can increase in the short term even when you are experiencing a long-term slowdown of hair loss (and a long-term increase in hair growth) while on a treatment that controls hair loss.
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