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.
Sinclair Scale: The 5-point Sinclair Scale is a modified visual grading scale. Grade 1 is normal. This pattern is found in all girls prior to puberty, but in only half of women age 80 or over. Grade 2 shows a widening of the central part. Grade 3 shows a widening of the central part and thinning of the hair on either side of the central part. Grade 4 reveals the emergence of diffuse hair loss over the top of the scalp. Grade 5 indicates advanced hair loss. This grade is uncommon, occurring in less than 1% of women.
Other drugs commonly attributed to causing madarosis are miotics, anticoagulants, anti-cholesterol drugs, antithyroid drugs, propranolol, valproic acid, boric acid, and bromocriptine.[21,99] Anticoagulants in high doses have been found to produce loss of scalp, pubic, axillary, and facial hair with loss of eyebrows after a latent period of a few weeks of treatment with dextran and heparin. Propranolol can cause diffuse alopecia along with loss of eyebrows due to telogen effluvium, usually after three months of therapy. Loss of medial aspect of eyebrows can be seen in fetuses exposed to valproic acid. Diffuse alopecia including that of eyebrows has been described due to chronic ingestion of mouthwashes containing boric acid. There was complete reversal following stopping the practice. Levodopa has been noted to cause severe diffuse alopecia within three months of daily use. Hair loss can occur soon after starting topical minoxidil therapy (due to detachment of club hairs following resting hairs reentering anagen), and after cessation of therapy (due to telogen effluvium).
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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.