If you’re stressed out on a regular basis, you may experience excessive hair shedding. Though this is not typically a permanent problem, physical and emotional stress can cause your eyebrow hair follicles to go into a resting phase. After resting phase, follicles enter telogen, or shedding phase, and will begin to shed. Usually, there is a delay between the stress and the shedding phase, sometimes up to 3-4 months. Thankfully, your hair growth should go back to normal again after your stress decreases. However, it can take several months.

If, however, you aren’t so sure that any of the causes listed above are a problem for you, then a good place to start may be to talk with your healthcare provider about testing. It is important to learn the root cause – especially if dealing with infertility – because that which is causing hair thinning and shedding may also be contributing to your inability to conceive.
Monitor any hair loss when starting new medications. Numerous medications can cause temporary or permanent hair loss, including chemotherapy,[28] antidepressants, and mood stabilizers.[29] Nonetheless, many other medications can cause hair loss in isolated cases. If you've noticed hair loss after starting a new medication, talk to your doctor about this side effect. 

Hansen’s disease – Hansen’s disease, popularly known as leprosy could cause loss of eyebrows. This condition is cause by infection of a bacterial known as mycobacterium leprae and if it affects your eyebrows region, you expect to lose some eyebrows i.e. if it affects your eyebrow skin region, a place it often afflicts, you could suffer from permanent eyebrow hair loss as ‘Tulay Cakiner-Egilmez, Ph.D., R.N’ reports in ‘Journal of the American Society of Ophthalmic Registered Nurses’. This and other infections that can cause loss of eyebrows.
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]
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