An underactive thyroid, known medically as hypothyroidism, is a condition where your thyroid gland doesn’t produce enough hormones to keep your metabolism running at its normal pace. It affects roughly 5 to 8 percent of the U.S. population, with women making up about 80 percent of diagnosed cases. Because the thyroid influences nearly every system in your body, low hormone levels can cause wide-ranging symptoms from fatigue and weight gain to depression and joint pain.
What Your Thyroid Actually Does
Your thyroid is a small, butterfly-shaped gland at the front of your neck. It releases two hormones: thyroxine (T4) and triiodothyronine (T3). T4 is the one produced in larger quantities, but it’s largely inactive on its own. Once T4 enters your bloodstream, cells throughout your body convert it into T3, the active form that directly affects how your cells function.
Together, T3 and T4 control how fast your body burns calories, how quickly your heart beats, how warm you feel, and how efficiently your brain processes information. They touch virtually every organ system. When levels drop too low, everything slows down: your metabolism, your heart rate, your digestion, even your thinking. That slowdown is the core of what makes hypothyroidism feel the way it does.
Common Causes
The most common cause by far is Hashimoto’s disease, an autoimmune condition where your immune system mistakenly attacks thyroid tissue and gradually destroys its ability to produce hormones. Over months or years, the gland produces less and less until symptoms develop.
Other causes include surgical removal of part or all of the thyroid (often for cancer or nodules), radiation treatment to the neck area, and certain medications that interfere with thyroid function. Thyroiditis, a general inflammation of the gland, can also lead to long-term underproduction after an initial flare. Some people are born with a thyroid that doesn’t work properly, a condition called congenital hypothyroidism. In rarer cases, a problem with the pituitary gland in the brain, which signals the thyroid to produce hormones, is responsible. Too much or too little iodine in the diet can also play a role, though iodine deficiency is uncommon in countries where salt is iodized.
Who Gets It
Hypothyroidism becomes significantly more common with age. In national survey data, people between 45 and 69 were about seven times more likely to be on thyroid medication than those aged 20 to 44, and people over 70 were eleven times more likely. Among those being treated, roughly 82 percent were women and 18 percent were men. That gender gap narrows somewhat for milder, untreated cases, where men make up closer to 45 percent of those affected.
The proportion of the U.S. population taking thyroid hormone medication nearly doubled between 1997 and 2016, rising from about 4 percent to 8 percent. Some of that increase reflects better detection rather than a true rise in the disease itself.
Symptoms to Recognize
Hypothyroidism symptoms develop gradually, which is part of why the condition often goes undiagnosed for months or years. Many people attribute early signs to aging, stress, or simply being out of shape. The most common symptoms include:
- Fatigue that doesn’t improve with sleep
- Unexplained weight gain or difficulty losing weight
- Cold sensitivity, feeling chilled when others are comfortable
- Joint and muscle pain
- Dry skin and thinning hair
- Heavy or irregular menstrual periods
- Fertility problems
- Slowed heart rate
- Depression or low mood
No single symptom confirms hypothyroidism on its own. Many of these overlap with other conditions, which is why blood tests are essential for an accurate diagnosis. Some people have only one or two noticeable symptoms, while others experience several at once.
How It’s Diagnosed
Diagnosis relies on a blood test measuring TSH, or thyroid-stimulating hormone. TSH is produced by the pituitary gland and tells your thyroid to make more hormones. When the thyroid underperforms, the pituitary responds by pumping out extra TSH, so a high TSH level is the primary signal that something is off.
The normal upper limit for TSH is generally around 4 to 5 mIU/L, though this varies somewhat by lab and by age. In younger adults (20 to 29), the upper boundary of normal may be as low as 3.5 mIU/L, while in people over 80, levels up to about 7.5 mIU/L can be normal. A free T4 test is typically run alongside TSH to confirm how much active hormone is circulating.
If your TSH is elevated but your T4 level is still in the normal range, you may have what’s called subclinical hypothyroidism. This is a milder form where the thyroid is struggling but still keeping up for now. Treatment decisions for subclinical cases are individualized. When TSH is above 10 mIU/L, treatment is generally recommended. Below that threshold, the decision depends on whether you have symptoms, your age, and other health factors.
Treatment With Thyroid Hormone
The standard treatment is a daily pill of synthetic thyroid hormone (levothyroxine), which replaces the T4 your thyroid isn’t making. Your body converts it to T3 just as it would natural hormone. The dose is calculated based on your lean body weight, typically around 1.6 micrograms per kilogram per day, though your starting dose may be lower if you’re older or have heart concerns.
Most people feel noticeably better within a few weeks of starting treatment, though it can take six to eight weeks for levels to fully stabilize. Your doctor will recheck your TSH after that initial period and adjust the dose if needed. Once you’re on the right dose, you’ll likely need periodic blood tests (usually once or twice a year) to make sure levels remain stable, since your needs can shift with age, weight changes, or pregnancy.
For people who are overweight, dosing based on actual body weight can lead to overtreatment. Current guidelines recommend basing the calculation on ideal body weight instead. Finding the right dose is a process of fine-tuning rather than a one-time decision.
What Happens If It Goes Untreated
Left untreated for years, hypothyroidism places increasing strain on the cardiovascular system, raises cholesterol levels, and can contribute to nerve damage and infertility. The most severe complication is myxedema coma, a rare but life-threatening emergency that occurs when the body has been deprived of thyroid hormone for a prolonged period. Symptoms include confusion, dangerously low body temperature, slow breathing, and swelling. It is fatal in 20 to 60 percent of cases, even with treatment. This outcome is almost entirely preventable with routine thyroid hormone replacement.
Iodine Supplements and Diet
Because the thyroid needs iodine to produce hormones, many people wonder whether iodine supplements would help. In most cases, they won’t. If iodine deficiency isn’t the cause of your hypothyroidism (and in developed countries, it usually isn’t), supplemental iodine provides no benefit. In fact, getting too much iodine from supplements, kelp tablets, or seaweed extracts can actually worsen hypothyroidism or trigger new thyroid problems. Unless you’ve been specifically told you’re iodine deficient, supplements are best avoided.
A balanced diet that includes seafood, dairy, and iodized salt provides enough iodine for normal thyroid function. No specific food has been shown to reverse hypothyroidism once it’s established, particularly when the cause is autoimmune. Treatment with thyroid hormone medication remains the most reliable and effective approach.