Your thyroid is a small gland in your neck that controls how your body uses energy. It’s only about 2 inches long, but it influences nearly every organ you have, from your heart and brain to your muscles and bones. When it works properly, you never think about it. When it doesn’t, the effects can show up in ways you might not expect, like unexplained weight changes, fatigue, or feeling too hot or too cold.
Where Your Thyroid Is and What It Looks Like
The thyroid sits at the front of your neck, just below your voice box and above your collarbones, wrapped around your windpipe. It’s shaped like a butterfly: narrow in the middle with two wider lobes that extend around each side of your throat. Despite its small size, it’s one of the most metabolically active organs in your body, and it has a higher concentration of the mineral selenium than any other organ.
What Your Thyroid Does
The thyroid’s primary job is producing hormones that regulate your metabolism. It makes two main hormones. The first, called T4, is essentially a storage form. Your thyroid releases T4 into the bloodstream, and then your liver, kidneys, and other tissues convert it into T3, the active form. T3 is what actually does the work: it helps control your heart rate, digestive function, brain development, muscle movement, and bone maintenance.
At the cellular level, these hormones tell your cells how much energy to burn. They regulate how your body breaks down fat, uses sugar, and produces heat. When you’re cold, your muscles respond through thyroid-related mechanisms that increase energy production and generate warmth. Your thyroid also plays a role in how quickly your body cycles through nutrients, speeding up both the building and breaking down of fats and sugars simultaneously.
The thyroid produces a third, lesser-known hormone called calcitonin, which helps manage calcium levels in your blood. It works by slowing down the cells that break down bone, which keeps calcium from flooding into your bloodstream. It also reduces how much calcium your kidneys reabsorb. This makes calcitonin a quiet but important player in bone health.
How Your Brain Controls the Thyroid
Your thyroid doesn’t operate on its own. It’s part of a communication loop between your brain and your neck. A region deep in your brain called the hypothalamus sends a signal to the pituitary gland (a pea-sized gland at the base of your brain), which then releases a hormone called TSH (thyroid-stimulating hormone). TSH tells your thyroid to produce and release T3 and T4.
Once thyroid hormone levels in your blood rise high enough, your brain detects this and dials back the signals. If levels drop, the brain ramps them up again. This negative feedback loop keeps your metabolism in a stable range. When something disrupts this loop, whether from disease, inflammation, or nutrient deficiency, thyroid problems develop.
What Your Thyroid Needs to Function
Two nutrients are especially critical for thyroid health: iodine and selenium. Your thyroid uses iodine as a raw building block for making its hormones. Without enough iodine, the gland simply can’t produce adequate T3 and T4. Most people in the U.S. get sufficient iodine through iodized salt and dairy products, but deficiency remains a concern in parts of the world without fortified food supplies.
Selenium plays a different but equally important role. Enzymes that contain selenium are responsible for converting the inactive T4 into the active T3. Selenium-based proteins also protect the thyroid gland from damage during hormone production, which generates hydrogen peroxide as a byproduct. Adults need about 55 micrograms of selenium per day, an amount easily found in a single Brazil nut or a serving of fish, eggs, or whole grains.
Signs of an Underactive Thyroid
When the thyroid doesn’t produce enough hormones, the condition is called hypothyroidism. Nearly 5 out of 100 Americans ages 12 and older have it, though most cases are mild. Because your metabolism slows down, the symptoms tend to creep in gradually. Common signs include fatigue, weight gain, feeling unusually cold, joint and muscle pain, dry skin, thinning hair, depression, and a slowed heart rate. Women may notice heavier or irregular periods, or difficulty getting pregnant.
These symptoms overlap with many other conditions, which is why hypothyroidism often goes undiagnosed for months or years. A blood test measuring TSH is the standard way to identify it. When the thyroid is underperforming, TSH rises because the brain is trying harder to stimulate hormone production.
Signs of an Overactive Thyroid
The opposite problem, hyperthyroidism, occurs when the thyroid produces too much hormone. The most common cause is Graves’ disease, an autoimmune condition in which the immune system attacks the thyroid and pushes it into overdrive. Symptoms reflect a metabolism running too fast: nervousness, irritability, weight loss, rapid or irregular heartbeat, muscle weakness, trouble sleeping, trembling hands, frequent bowel movements, and difficulty tolerating heat. Some people develop a visible swelling in the neck called a goiter.
In hyperthyroidism, TSH levels drop because the brain is trying to slow things down. Blood tests for TSH, T3, T4, and thyroid antibodies help confirm the diagnosis and identify the underlying cause.
Thyroid Nodules
Thyroid nodules are lumps that form within the gland. They’re surprisingly common. About 4 to 7 percent of people have nodules large enough to feel by touch, but when doctors look with ultrasound, they find nodules in 19 to 67 percent of people. The vast majority are harmless. Roughly 5 to 10 percent of palpable nodules turn out to be cancerous, and thyroid cancer is one of the more treatable cancers when caught early.
How to Check Your Thyroid at Home
You can do a simple visual check with a handheld mirror and a glass of water. Hold the mirror so you can see the lower front of your neck, between your collarbones and your voice box. Tip your head back, take a sip of water, and swallow while watching that area in the mirror. Look for any bulges or protrusions that appear as you swallow. The thyroid sits below the Adam’s apple, closer to the collarbones, so don’t confuse the two. Repeat a few times to get a clear look. If you notice anything unusual, a doctor can follow up with a physical exam and imaging.