Hypothyroidism is a condition where your thyroid gland doesn’t produce enough hormones to meet your body’s needs. It affects roughly 10% of the U.S. adult population when you include both overt and milder forms, and it’s about twice as common in women as in men. Because thyroid hormones influence nearly every organ system, from your metabolism to your mood, an underactive thyroid can produce a wide range of symptoms that often develop so gradually you don’t notice them for months or years.
How Your Thyroid Works
Your thyroid is a small, butterfly-shaped gland at the front of your neck. Its job is to produce two hormones: T4 (thyroxine) and T3 (triiodothyronine). About 80% of what the thyroid releases is T4, which is relatively inactive on its own. Your body converts T4 into T3, the form that actually drives cellular activity, in tissues throughout the body.
The whole system runs on a feedback loop involving your brain. Your hypothalamus, a structure deep in the brain, sends a signal to your pituitary gland, which responds by releasing thyroid-stimulating hormone (TSH) into your bloodstream. TSH tells the thyroid to make and release T4 and T3. When hormone levels rise high enough, the brain dials back TSH production. When levels drop, TSH goes up to push the thyroid harder. In hypothyroidism, this loop is disrupted: the thyroid can’t keep up, so TSH climbs higher and higher trying to compensate.
What Causes It
In developed countries, the most common cause is Hashimoto’s thyroiditis, an autoimmune condition where the immune system attacks and gradually destroys thyroid tissue. In Hashimoto’s, certain white blood cells infiltrate the thyroid, causing chronic inflammation and scarring that steadily reduces the gland’s ability to produce hormones. Over 90% of people with Hashimoto’s have detectable antibodies against a key thyroid enzyme, and 50% to 80% have antibodies targeting the protein the thyroid uses to build its hormones.
Globally, iodine deficiency remains a major cause. Your thyroid needs iodine as a raw ingredient to manufacture hormones. Roughly 2 billion people worldwide have insufficient iodine intake, and about 50 million of them show clinical effects. In countries that add iodine to table salt (including the U.S.), this is rarely a problem, but in parts of Asia, Africa, and Eastern Europe, iodine deficiency still drives high rates of thyroid disease. Interestingly, too much iodine can also suppress thyroid function, which is why populations in coastal Japan and parts of China that consume large amounts of seaweed or iodine-rich water have elevated rates of hypothyroidism as well.
Other causes include surgical removal of part or all of the thyroid (often for cancer or an overactive thyroid), radiation therapy to the head and neck, and certain medications. Lithium, commonly prescribed for bipolar disorder, is one of the better-known drug-related triggers. Less commonly, hypothyroidism results from problems with the pituitary gland itself, which can’t produce enough TSH to stimulate the thyroid. This is called secondary hypothyroidism and can occur after pituitary surgery, tumors, or head injuries.
Symptoms to Recognize
Because thyroid hormones regulate your metabolism, an underactive thyroid tends to slow everything down. The most common symptoms include:
- Fatigue that doesn’t improve with rest
- Weight gain or difficulty losing weight, typically 5 to 15 pounds
- Cold sensitivity, feeling chilly when others are comfortable
- Dry skin and thinning hair
- Joint and muscle pain
- Heavy or irregular menstrual periods
- Slowed heart rate
- Depression or low mood
These symptoms overlap with many other conditions, which is part of why hypothyroidism often goes undiagnosed for a while. Many people attribute the fatigue and weight gain to aging, stress, or poor sleep. The onset is usually gradual over months to years as the thyroid slowly loses function, so there’s rarely a single moment where you feel a clear shift.
How It’s Diagnosed
A simple blood test measuring TSH is the standard first step. Normal TSH ranges vary slightly depending on the lab, but most reference ranges fall roughly between 0.3 and 4.5 to 5.0 mIU/L. When your thyroid is underperforming, TSH rises because the pituitary is working overtime trying to stimulate more hormone production. A TSH above the upper limit of your lab’s range, combined with low T4, confirms overt hypothyroidism.
There’s also a milder form called subclinical hypothyroidism, where TSH is elevated (typically between 5 and 10 mIU/L) but T4 remains in the normal range. You may or may not have noticeable symptoms at this stage. Providers sometimes categorize this further: grade 1 (TSH between 4.5 and 9.9) and grade 2 (TSH of 10 or higher). Grade 2 is more likely to progress to full hypothyroidism and is more often treated. Whether to treat grade 1 depends on your symptoms, age, and other risk factors.
If Hashimoto’s is suspected, your provider may also test for thyroid antibodies. These aren’t required for diagnosis, but they help explain the underlying cause and predict whether subclinical hypothyroidism is likely to worsen over time.
Who Is Most Affected
Hypothyroidism becomes significantly more common with age. U.S. data shows a prevalence of about 3.6% in people ages 12 to 29, rising to 6.5% in the 30 to 49 age group, 15.3% among those 50 to 79, and roughly 31% in people over 80. Women make up about 76% of all diagnosed cases. The reasons for the strong sex difference aren’t fully understood, but the autoimmune component likely plays a role, since autoimmune conditions in general are more common in women.
Common coexisting conditions include cardiovascular disease, high blood pressure, and lipid disorders. This partly reflects the fact that hypothyroidism tends to raise cholesterol and that it’s most prevalent in older adults who already carry these risks. Treating the thyroid problem often improves cholesterol numbers on its own.
Treatment and What to Expect
Treatment is straightforward: a daily pill of synthetic T4 (levothyroxine) replaces the hormone your thyroid isn’t making. For most otherwise healthy adults, the full replacement dose is about 1.6 micrograms per kilogram of body weight per day. For a 155-pound person, that works out to roughly 100 to 125 micrograms daily. Older adults or those with heart conditions start at a much lower dose and increase gradually.
You take the pill on an empty stomach, ideally 30 to 60 minutes before breakfast. This matters because absorption from the gut ranges from 40% to 80% depending on what else is in your stomach. High-fiber foods, soy products, walnuts, and grapefruit juice can all reduce how much of the medication your body actually takes in. Calcium and iron supplements are particularly notorious for blocking absorption and should be separated from your thyroid pill by at least four hours.
After starting or adjusting your dose, it takes four to six weeks to see the full effect. Your provider will recheck your TSH at that point and adjust the dose in small increments until your levels normalize. Most people need periodic monitoring, typically once or twice a year once a stable dose is established. The goal is a TSH within the normal range and, more importantly, resolution of your symptoms.
Many people notice improvements in energy and mood within two to three weeks of starting treatment, though some symptoms like dry skin and hair thinning take longer to resolve. Treatment is generally lifelong, especially when the cause is Hashimoto’s or thyroid removal, since the underlying problem doesn’t reverse.
When Hypothyroidism Becomes Dangerous
Left untreated for years, severe hypothyroidism can lead to a rare but life-threatening emergency called myxedema coma. Despite the name, actual coma is uncommon, but the condition involves dangerously low body temperature, slowed breathing, a drop in heart rate, swelling of the face and hands, and confusion or extreme lethargy progressing to unresponsiveness. It occurs at a rate of about 1 case per million people per year, but carries a mortality rate of roughly 30%, and as high as 60% even with treatment.
Myxedema coma is almost always triggered by an additional stressor, such as an infection, surgery, or exposure to cold, in someone whose hypothyroidism has gone untreated or severely undertreated for a long time. It’s vanishingly rare in anyone who is taking their medication and getting regular blood work. The more common long-term risks of poorly managed hypothyroidism are elevated cholesterol, heart disease, fertility problems, and in pregnant women, developmental issues for the baby.
Iodine and Your Thyroid
The recommended daily iodine intake for adults is 150 micrograms. Most people in the U.S. and other developed countries get enough through iodized salt, dairy, eggs, and seafood. Pregnant and breastfeeding women need more (around 150 to 249 micrograms) because iodine deficiency during pregnancy can cause serious and irreversible neurological problems in the developing baby.
If you already have Hashimoto’s or another form of autoimmune thyroid disease, taking high-dose iodine supplements can actually worsen thyroid function. Healthy adults can tolerate up to about 600 to 1,100 micrograms per day without problems, but the safe upper limit for someone with existing thyroid disease is considerably lower. Unless your provider has confirmed an iodine deficiency, supplementing beyond what a normal diet provides is unlikely to help and could do harm.