What Actually Causes Hypothyroidism? The Surprising Truth

Have you ever felt exhausted no matter how much you sleep, cold when everyone else is comfortable, or watched the scale creep up even though nothing changed? It’s easy to blame yourself, maybe you’re just lazy, or eating wrong. But here’s the twist: for millions of people, the real culprit is a small, butterfly-shaped gland in the neck that quietly stopped doing its job.

Understanding what actually causes hypothyroidism can change how you see your own body, and it’s probably not what you think.

Jump to a sectionIn this article10 sections

1. First, What Does the Thyroid Actually Do?
2. The Surprising Truth: It’s Usually Your Own Immune System
3. What About Iodine? The Global Picture
4. Other Real Causes You Should Know About
5. Causes vs. Myths: What’s Real and What’s Not
6. How Do Doctors Actually Figure It Out?
7. Actionable Tips: What You Can Actually Do
8. Frequently Asked Questions
9. You’re Not Imagining It, and It’s Not Your Fault
10. Sources

First, What Does the Thyroid Actually Do?

Your thyroid is a small gland at the front of your neck, shaped a bit like a butterfly. Think of it as your body’s thermostat and gas pedal rolled into one. It makes thyroid hormone, a chemical messenger that tells nearly every cell how fast to work.

From my coaching: When a client is tired, cold and stuck despite doing everything right, I encourage them to ask their doctor about a thyroid test. Once treatment is dialed in, most people can train and progress like anyone else.

When your thyroid makes enough hormone, everything hums along: your heart rate, your body temperature, how quickly you burn energy, even your mood. But when it makes too little, everything slows down. That slowdown is what we call hypothyroidism, also known as an underactive thyroid.

The tricky part? The slowdown happens gradually. Many people feel “off” for months or years before they realize something is medically wrong. Sound familiar?

The Surprising Truth: It’s Usually Your Own Immune System

Here’s the fact that catches most people off guard. In the U.S. and other places where people get enough iodine in their diet, the leading cause of hypothyroidism is an autoimmune disease called Hashimoto’s thyroiditis (named after the doctor who first described it).

Autoimmune means your immune system, the system that’s supposed to protect you, gets confused and starts attacking your own healthy tissue by mistake. In Hashimoto’s, the immune system slowly attacks the thyroid gland. Over time, the gland gets damaged and can’t make enough hormone.

Let that sink in. This isn’t about willpower, or eating the “wrong” foods, or not exercising enough. It’s your body’s defenses misfiring. You didn’t cause it, and you can’t simply “push through” it.

Who’s More Likely to Get Hashimoto’s?

While anyone can develop it, some patterns show up again and again. Research suggests it’s far more common in women than men, and the risk tends to rise with age. You may also be more likely to develop it if:

• Someone in your family has thyroid or autoimmune problems.
• You already have another autoimmune condition, like type 1 diabetes or celiac disease.
• You’ve recently been pregnant (more on that below).

Genetics load the gun, so to speak, and things in your environment may pull the trigger. Scientists are still studying exactly what those triggers are.

What About Iodine? The Global Picture

Now here’s where it gets interesting, and where a lot of bad advice comes from.

Around the world, the most common cause of hypothyroidism is actually iodine deficiency. Your thyroid needs iodine (a mineral found in foods like seafood, dairy, and iodized salt) as a building block to make hormone. Without enough, it simply can’t keep up.

This used to be a huge problem in the U.S. too. That’s why table salt was fortified with iodine decades ago: a public health win that quietly protected millions.

But here’s the myth to watch out for: more iodine is not automatically better. If you already get enough (which most people in developed countries do), loading up on iodine supplements can actually harm your thyroid, and in people with Hashimoto’s, excess iodine may make things worse.

So if you’ve seen influencers pushing high-dose iodine or kelp pills as a “thyroid cure,” that advice can do real damage. Please talk to a doctor before adding iodine.

Other Real Causes You Should Know About

Autoimmune disease and iodine problems get most of the attention, but they’re not the whole story. Here are other genuine causes.

Thyroid Surgery or Removal

Sometimes the thyroid has to be partly or fully removed, because of nodules, an overactive thyroid, or thyroid cancer. If enough of the gland is gone, it can’t make enough hormone, and hypothyroidism follows.

Radiation Treatment

Certain treatments, including radioactive iodine therapy (used to treat an overactive thyroid) and radiation aimed at the head or neck for cancer, can damage the thyroid over time. This is a known and expected side effect that doctors watch for.

Certain Medications

A handful of medicines can interfere with how your thyroid works. Two well-known examples are lithium (used for some mood conditions) and amiodarone (a heart medication). If you take these, your doctor will usually keep an eye on your thyroid with routine blood tests.

Problems From Birth (Congenital Hypothyroidism)

Some babies are born with a thyroid that didn’t form properly or doesn’t work well. This is why newborns in many countries get a heel-prick blood test shortly after birth: catching it early protects healthy growth and brain development.

A Rare Cause: The Pituitary Gland

In a small number of cases, the thyroid itself is fine, but the pituitary gland, a tiny gland in the brain that tells the thyroid what to do, isn’t sending the right signals. This is uncommon, but it’s why doctors sometimes look beyond the thyroid itself.

Pregnancy and the Months After

Some women develop postpartum thyroiditis: thyroid inflammation in the year after giving birth. For some it’s temporary; for others it can lead to lasting hypothyroidism. New mom feeling wiped out beyond the usual exhaustion? It’s worth mentioning to your doctor rather than brushing off.

Causes vs. Myths: What’s Real and What’s Not

With so much noise online, it helps to sort fact from fiction. Here’s a quick side-by-side.

Real, Established Causes	Common Myths & Overblown Claims
Hashimoto’s (autoimmune) thyroiditis	“You gave yourself hypothyroidism by being lazy”
Iodine deficiency (mainly worldwide)	“Megadosing iodine or kelp will cure it”
Thyroid surgery or removal	“Eating broccoli, kale, or soy causes it”
Radiation and radioactive iodine therapy	“It’s just a slow metabolism you can willpower away”
Certain medications (e.g., lithium)	“A special detox or cleanse will fix your thyroid”
Congenital (from-birth) thyroid problems	“Adrenal fatigue is behind your thyroid issues”

A quick word on that soy-and-broccoli myth, since it worries so many people. Foods like broccoli, cabbage, kale, and soy contain natural compounds called goitrogens that can affect the thyroid, but realistically, only in very large amounts, and mostly if you’re also low on iodine. For the vast majority of people, eating normal servings of these healthy vegetables is completely fine.

Cooking them reduces those compounds even further. So don’t ditch your veggies over a scary headline.

How Do Doctors Actually Figure It Out?

Since the symptoms of hypothyroidism (tiredness, weight gain, feeling cold, dry skin, low mood, constipation, thinning hair) overlap with so many other conditions, you truly can’t diagnose it by symptoms alone. Ever notice how “tired and gaining weight” could describe a hundred different problems?

That’s why the gold standard is a simple blood test. The main one measures TSH (thyroid-stimulating hormone), the signal your brain sends to your thyroid. When the thyroid is underactive, TSH usually climbs as your body tries to shout louder. Your doctor may also check thyroid hormone levels directly and look for the antibodies that point to Hashimoto’s.

The good news? Once it’s identified, hypothyroidism is very manageable. It’s one of the most treatable long-term conditions out there.

Actionable Tips: What You Can Actually Do

You can’t always prevent hypothyroidism, especially the autoimmune kind, but you can be smart and proactive. Here’s how.

• Know your family history. If thyroid or autoimmune conditions run in your family, mention it to your doctor. It helps them know when to check.
• Don’t self-diagnose or self-treat. Feeling off is a reason to get tested, not a reason to start random supplements from the internet.
• Skip the iodine megadoses. Unless a doctor specifically recommends it, more iodine is not a safe DIY fix, and it can backfire.
• Use iodized salt in normal amounts. For everyday cooking, standard iodized salt helps cover your basic iodine needs without overdoing it.
• Get tested if symptoms linger. Persistent fatigue, unexplained weight changes, feeling cold, or low mood that won’t lift? A quick blood test can rule the thyroid in or out.
• If you’re pregnant or recently gave birth, flag new or worsening tiredness. Postpartum thyroid changes are real and treatable.
• Take prescribed thyroid medication consistently. If you’re diagnosed and treated, taking it as directed, and rechecking levels over time, keeps you feeling your best.

Frequently Asked Questions

You’re Not Imagining It, and It’s Not Your Fault

If there’s one thing to carry away, it’s this: hypothyroidism is a real, physical condition with real, understandable causes. It’s not a personal failing, and it’s not something you talked yourself into. Most of the time, it comes down to your own immune system or a gland that simply needs support, not anything you did.

So if your body has felt slow, heavy, and unlike itself, be kind to yourself and get curious instead of critical. The surprising truth about what causes hypothyroidism is also a hopeful one: once you know what’s really going on, it’s one of the most manageable conditions you can face.

A simple conversation with your doctor and a quick blood test could be the first step toward feeling like you again. You deserve that.

Sources

These are the references behind the main facts in this article, if you want to read further:

• NIDDK: Hypothyroidism
• NIDDK: Hashimoto's Disease
• MedlinePlus: Hypothyroidism
• American Thyroid Association: Hypothyroidism

This article is for general education and isn’t medical advice. I’m a NASM-certified personal trainer and NESTA nutrition coach, not a physician. Only your healthcare provider can diagnose and treat a thyroid condition.

See your doctor if you have ongoing fatigue, feel cold all the time, have unexplained weight gain, or notice swelling in your neck. This article is not a substitute for personal care.

Keep going: this article is part of my natural healing and healthy living guide. For every guide, free tool, and honest review, organized by where you are stuck, visit the Hormone Health Hub.