The 52-year-old documentary maker gave the update on social media, explaining that the loss around the sides of his mouth was the reason he had not grown his beard as much recently.
In a post on Instagram, Theroux shared a series of photos highlighting how his facial hair has changed in recent months.
In the accompanying caption he told his 763,000 followers: “So this is what my beard grows like now due to what I think is probably alopecia.
“Basically I get a little triangle of stubble around my mouth and some more at the sides."
While Theroux was keen to stress the facial loss was "not a big deal", he wanted to let fans know what was going on.
“It’s not a big deal," he wrote. "But I don’t want people to think I’m doing something creative with my facial hair and doing it badly, when it’s just what I look like when I don’t shave."
Theroux added: “This is also why I don’t wear a beard as much anymore. Maybe it’ll grow back. Who knows?
“It mainly happened over the course of 2022. You get a sense of how it progressed from the other two photos taken six months before and a year before.”
What is alopecia areata?
According to the charity Alopecia UK, alopecia areata is believed to be an autoimmune condition, which often starts with isolated patches of hair loss.
The patches, commonly in one or more coin-sized areas, are typically found on the scalp and/or across the body including the beard, eyebrows, eyelashes or body hair, including pubic hair.
"In alopecia areata, cells from the immune system (a specific type of T cell, known as NKG2D+ T cells) gather around the hair follicles," the charity explains.
"These cells attack the follicle, stopping it from producing more hair. The exact way in which this happens is not yet understood."
Watch: Prince Harry ‘alarmed’ by William’s hair loss
What is alopecia barbae?
Alopecia barbae is a specific form of alopecia areata, in which hair loss affects the beard and moustache.
"Usually, it comes on suddenly and you begin losing beard hair in small circular patches," explains Anabel Kingsley, brand president and trichologist at Philip Kingsley.
While alopecia barbae can occur only in the beard area, it can also happen alongside hair loss in other areas, such as the scalp.
Alopecia barbae usually causes beard hair to fall out in small circular patches. If more hair is falls out, the circles can begin to overlap.
According to Alopecia UK, hair around the edge of the patches sometimes turns white.
What are the symptoms?
In some people alopecia barbae can appear gradually, while for others it can come on quite suddenly.
One of the first symptoms, according to Gillette UK, are small bald circular patches in your beard that gradually get larger in size.
The skin around these bald patches may also start to feel itchy or painful before you start losing your hair and some people can experience skin irritation, inflammation and redness in the patchy bald areas.
Treatments for alopecia areata
According to Alopecia UK, there is currently no cure for alopecia areata, however, there are a variety of treatments to try, although none are guaranteed to work.
Many people experience spontaneous regrowth without treatment and treatments are more likely to be effective in milder cases of alopecia areata, with small patches of hair loss.
Common treatments for alopecia areata include corticosteroids (designed to dampen down the immune response) and topical immunotherapy (to distract the immune response).
There are also some cosmetic treatments including beard transplants, with one Harley Street surgeon reporting a spike in beard transplant enquiries recently.
Harley Street surgeon Dr Asim Shahmalak from Crown Clinic claims he has had a 25% rise in bookings in recent years.
Dr Shahmalak said: “A proper, full-grown beard has been the ultimate definition of masculinity for centuries.
“But the sad truth is that many men in Britain struggle to grow beards, sideburns and moustaches.
“They have significant gaps in their facial hair and the end result can look patchy and unconvincing.
Dr Shahmalak claimed more men are turning to surgery to get their facial hair bolstered with hair transplants so that they have a fuller beard with no gaps.