If something is upsetting your stomach, you’re pretty aware of it because you’ll have a stomach ache. Or if you you’re coming down with a strep throat, you know your throat is affected if it’s sore. But having something wrong with your thyroid might be a little less obvious.

As many as 30 million Americans have a thyroid disorder, but only about half of those people are diagnosed, according to The American Association of Clinical Endocrinologists. That means 15 million people are living with a thyroid disorder and they don’t even know it.

What even is your thyroid?

Quick health lesson: Your thyroid is a butterfly-shaped gland located in your neck. It’s what’s responsible for telling the other cells in your body to function in a way and at a rate to keep your body happy and healthy and acting as it should. It’s a relatively small glad, but it actually controls a lot in your body.

If your thyroid isn’t functioning properly, it can have a big impact on your health. Your thyroid could be overactive (known as hyperthyroidism), which is when you’re producing too much hormone, or underactive (known as hypothyroidism), when not enough hormone is produced.

Both of these conditions are what most often result in underlying thyroid disease. And that’s when the problems can creep in. So how do you know if what you’re experiencing is because of your thyroid, or something else?

Here are a few ways to tell your thyroid may be out of whack:

1. You’re super tired.

   Are you just constantly exhausted every day, and it’s not from any particular reason? Sort of like you have crazy bad jet lag, but you didn’t recently make a big trip anywhere? While having little energy can be a sign of a lot of things, it’s a big one when it comes to thyroid disease.

   “Fatigue is the number one symptom I see. It’s the kind of fatigue where you’re still tired in the morning after a full night’s sleep—that’s a clue that you’re not simply sleep deprived; your thyroid may be underactive,” says Robin Miller, MD, a board-certified internist and co-author of The Smart Woman’s Guide to MidLife & Beyond.

2. You feel unusually depressed.

   Thyroid disease is actually sometimes misdiagnosed as depression. You might feel sad, hopeless, and like you just don’t want to get out of bed in the morning. That’s because hypothyroidism is said to impact serotonin (the “feel good”) hormone in the brain, which can make you feel pretty down.

3. You’re pretty jittery and anxious.

   On the other hand of feeling blue, you might be feeling crazy wired and frazzled if you’re experiencing hyperthyroidism. That’s because an overactive thyroid can have an impact on your metabolism, which can put your whole body into overdrive. So basically, if you didn’t drink a cup of joe but feel like you had a jug of it, your thyroid might be to blame.

4. You have extreme hunger (or aversions to food).

   If your appetite feels like it’s on overdrive—like wanting to eat one meal after the other—you could be experiencing hyperthyroidism. On the contrary, hypothyroidism can do weird things to your sense of taste and smell, so you might not necessarily want to eat the foods you usually enjoy.

Curious about the other signs that could mean you’re having an issue with your thyroid? Check out the video below for more symptoms!