Last night, Oscar nominee Will Smith smacked presenter Chris Rock after the latter made a joke on stage about the former’s wife having a shaved head. “Jada, can’t wait for G.I. Jane 2,” Rock said to laughter from the room, but an unamused look from Jada Pinkett Smith herself. And then things got ugly. Smith walked on stage and smacked Rock and then sat back down, yelling “Keep my wife’s name out your” bleeping “mouth,” twice.

Why the anger? One suspected reason is, Jada Pinkett Smith has alopecia, a hair-loss-causing autoimmune disorder she has spoken openly about before. In 2018, the host revealed her condition on her show Red Table Talk. She first noticed clumps of hair falling out in the shower. “It was one of those times in my life where I was literally shaking with fear,” she said. “That’s why I cut my hair and continued to cut it.” She shaved it completely last year. “Willow made me do it because it was time to let go,” she wrote in an Instagram caption, referencing her daughter. “BUT … my 50’s are bout to be divinely lit with this shed (shaved head).”

What causes sudden hair loss to happen? In short: a lot of things. Some are known culprits—like genes—and other reasons are a mystery. While you can’t do anything about your genetics, you can do something about one reason why you might be losing hair. Read on to find out what—and to ensure your health and the health of others, don’t miss these Sure Signs You’ve Already Had COVID.

Androgenic alopecia is the scientific term for common hair loss. If you have it, join the (hair) club: it affects roughly half of all men and women. Caucasians are most impacted, followed by Asians and African Americans, and then other groups. And in a twisted version of form following function, the incidence approximates the age in white men, with 50% affected by age 50. In women, the chance of experiencing hair loss increases after menopause.

A strand lasts about two to three years on your head, after which it falls out and is replaced with a new one, in a months long cycle. We all lose hairs routinely, shedding as many as 100 strands of hair a day—and usually, these hairs are replaced over time. But when a hair falls out and is not replaced—or is replaced with a much thinner strand—the march of hair loss has begun. If it continues, we go bald.

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The most common cause of hair loss is hereditary—thanks gramps! For years, the myth prevailed that the genes for male pattern baldness (which also affects women, albeit differently) are passed from mother to son on her X chromosome. Conventional wisdom held that men could basically look at the hairdos (or lack thereof) on their mother’s side and get a pretty good indication of how they might end up looking. But doctors now say it’s more correct to blame both your parents for thinning hair.

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Heredity, hormonal changes, medications, and various health conditions are often to blame. But there’s one other reason that could surprise you.

It’s your thyroid.

The butterfly-shaped gland that sits at the base of your throat is responsible for releasing hormones that control metabolism—governing how your body uses energy. The thyroid’s hormones regulate vital body functions like breathing, heart rate, body weight, body temperature, your nervous system, and so much more. Simply put: it’s essential.6254a4d1642c605c54bf1cab17d50f1e

Here’s a lesser-known fact: thyroid hormones are necessary for the development and maintenance of the hair follicle—which is why so many people with a thyroid dysfunction shed hair. Sometimes the thyroid under or over-produces that hormone—which can wreak havoc on your system—and can impact the development of hair at the root, affecting hair growth. The result? Thinning or baldness, depending on how your thyroid condition goes undiagnosed and untreated.

Common autoimmune thyroid conditions like Hashimoto’s thyroiditis and Graves’ disease can result in hair loss. To add some insult to injury, hair on other parts of your body may also be affected, such as body hair, and yes, eyebrows—with thyroid dysfunction. This is known as diffuse hair loss and is sometimes the presenting symptom for low or hypothyroidism.

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Here’s some inside scoop from Dr. Joseph Feuerstein, Director of Integrated Medicine at Stamford Hospital, and Associate Professor of Medicine at Columbia University:

“Most conventional doctors are not on the lookout for subclinical hypothyroidism—a real condition that affects millions of unsuspecting people. With subclinical hypothyroidism, your thyroid is underactive, but your levels are still in a ‘normal’ range. Your thyroid is struggling to keep up, and you’ll start having symptoms like fatigue, constipation, weight gain—and possibly hair loss. If left untreated, this will become full-blown hypothyroidism.”

The take-away is that you might have thyroid dysfunction that’s not developed enough for your doctor to take notice unless they have a more nuanced integrative practice—so be proactive and ask about subclinical hypothyroidism, especially if you’re experiencing hair loss!

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Most cases of thyroid-related hair loss are temporary and treatable! Hair loss from thyroid dysfunction is typically reversed after your thyroid hormone levels stabilize. The less good news? This could take some time—as in months, and it may be incomplete.

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Here’s what you can do to prevent this from happening in the first place: Have a check-up every year, and have your thyroid levels checked so that you can catch any irregularity quickly and address it. Ask your doctor to check TSH levels along with thyroid hormone in case you have subclinically low thyroid that can develop later into more full-blown hypothyroidism.

In the meantime, keep further hair loss at bay by treating the hair you still have as well as possible. Avoid excessive brushing; harsh coloring products (we’re looking at you bleach); and hairstyles that pull tightly like slicked-back buns. If you’re feeling self-conscious about your thinning hair or bald patches, a pretty silk scarf or stylish wig might be something to consider while your hair grows back. And to protect your life and the lives of others, don’t visit any of these 35 Places You’re Most Likely to Catch COVID.