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Medically reviewed by the HealthVetted Medical Board
Hypothyroidism (underactive thyroid) is a condition in which the thyroid gland does not make enough thyroid hormone to meet the body's needs, slowing down many of the body's functions. Because thyroid hormone influences nearly every organ system, low levels can cause fatigue, weight gain, cold intolerance, and a wide range of other symptoms that often develop gradually.
The thyroid is a small, butterfly-shaped gland at the front of the neck. It produces two main hormones — thyroxine (T4) and triiodothyronine (T3) — that act as the body's metabolic thermostat, regulating how fast cells use energy. This affects heart rate, body temperature, digestion, mood, and many other processes (NIDDK).
Thyroid hormone production is controlled by a feedback loop. The pituitary gland in the brain releases thyroid-stimulating hormone (TSH), which signals the thyroid to make more hormone. When thyroid hormone levels drop, the pituitary secretes more TSH to push the gland harder. This is why an elevated TSH is the hallmark laboratory finding in most hypothyroidism — the brain is calling for more hormone than the thyroid can supply (StatPearls).
Hypothyroidism is classified by where the problem originates:
Hypothyroidism affects roughly 5 in 100 Americans aged 12 and older, and it is far more common in women than in men (NIDDK). It becomes more frequent with age.
In the United States and other regions with adequate dietary iodine, the leading cause is Hashimoto's thyroiditis — an autoimmune disease in which the immune system gradually attacks and damages the thyroid (NIDDK; StatPearls). Worldwide, however, iodine deficiency remains the most common cause of hypothyroidism overall, because iodine is an essential building block of thyroid hormone (StatPearls). Iodine deficiency is rare in the U.S. due to iodized salt.
Other recognized causes include:
Risk factors that raise the likelihood of developing hypothyroidism include (NIDDK):
Symptoms typically develop slowly, sometimes over months or years, and can be easy to attribute to aging, stress, or other conditions. Severity tracks loosely with how low hormone levels are. Common signs and symptoms include (NIDDK; StatPearls):
Many people have only a few of these, and some with mild or subclinical disease have no symptoms at all. Because the picture is nonspecific, diagnosis relies on blood tests rather than symptoms alone.
Hypothyroidism is diagnosed with blood tests, not symptoms alone (NIDDK).
Because a single reading can be skewed by acute illness or lab variation, clinicians frequently repeat an abnormal TSH before diagnosing or treating, particularly in mild cases. T3 testing is not routinely used to diagnose hypothyroidism. In suspected central hypothyroidism, TSH may be low or inappropriately normal despite a low free T4, which requires specialist evaluation.
Newborns are screened at birth, and many guidelines support testing adults who have symptoms or risk factors. Universal screening of all healthy adults is debated and not uniformly recommended.
Hypothyroidism is highly treatable. Treatment aims to restore normal thyroid hormone levels, relieve symptoms, and bring TSH back into the target range.
Lifestyle changes do not cure or replace medication for true hypothyroidism, but they support overall health: a balanced diet, regular activity, adequate sleep, and not over- or under-consuming iodine. In the U.S., most people get enough iodine from a normal diet, and people with Hashimoto's may be sensitive to excess iodine, so high-dose iodine or kelp supplements should be avoided unless a clinician advises otherwise (NIDDK). There is no proven dietary cure.
There is no OTC product that reliably treats hypothyroidism. "Thyroid support" supplements and glandular products are not substitutes for prescription therapy; some contain undisclosed thyroid hormone or iodine and can be harmful. Anyone considering supplements should discuss them with a clinician, especially because iron, calcium, and antacids can interfere with thyroid medication (see below).
The standard, first-line treatment is levothyroxine, a synthetic form of the T4 hormone that the body converts to active T3 as needed. It is taken once daily and is one of the most commonly prescribed medications in the U.S. (StatPearls). Key points about levothyroxine therapy:
Other prescription options include liothyronine (synthetic T3) and desiccated (natural) thyroid extract, which contain T3. Combination or T3-containing therapy is sometimes considered for people who feel unwell on levothyroxine alone, but evidence that it is superior is mixed, and levothyroxine remains first-line.
Whether to treat subclinical hypothyroidism is individualized. Treatment is more likely to be recommended when TSH is markedly elevated (commonly cited as above ~10 mIU/L), when symptoms or TPO antibodies are present, or during pregnancy or pregnancy planning; milder cases are often monitored (StatPearls).
Most hypothyroidism — especially autoimmune Hashimoto's — cannot be prevented, because it stems from genetics and immune factors outside personal control. The clearest preventable cause globally is iodine deficiency, which is addressed at the population level through iodized salt.
What hypothyroidism responds to extremely well is long-term management. For most people it is a lifelong but very manageable condition, much like managing blood pressure. Sound management includes:
Pregnancy deserves special attention: thyroid hormone needs rise, untreated or undertreated hypothyroidism is linked to pregnancy complications and effects on fetal development, and dose requirements often increase early. Women who are pregnant or planning pregnancy should coordinate closely with their clinician (ACOG; Endocrine Society).
See a clinician if you have persistent, unexplained symptoms such as ongoing fatigue, weight gain, cold intolerance, dry skin, constipation, low mood, or a visibly enlarged thyroid — a simple blood test can clarify the cause. People with thyroid risk factors, those who are pregnant or planning pregnancy, and anyone with a known thyroid condition due for follow-up should also seek care.
Seek urgent or emergency care for these red flags, which can signal dangerously severe hypothyroidism (myxedema coma, a rare but life-threatening emergency that usually occurs in long-untreated disease):
Myxedema coma requires immediate hospital treatment (StatPearls). It is uncommon but underscores why hypothyroidism should not be left untreated.
The outlook is excellent with proper treatment. Levothyroxine effectively normalizes thyroid hormone levels in most people, and symptoms typically improve over weeks to a few months once the dose is correct. With consistent medication and periodic monitoring, the vast majority of people live full, normal lives (NIDDK).
Most people need treatment for life, because the underlying cause (such as Hashimoto's or surgery) is permanent; some cases from temporary thyroiditis or certain medications can resolve. Untreated or poorly controlled hypothyroidism, by contrast, can contribute to high cholesterol, heart problems, infertility and pregnancy complications, peripheral neuropathy, goiter, and — rarely — myxedema coma. The strong takeaway is that hypothyroidism is one of the most manageable chronic conditions when diagnosed and treated.
In most cases, yes. When the cause is permanent — such as Hashimoto's thyroiditis, thyroid removal, or radioactive iodine treatment — the thyroid cannot recover, so daily replacement is lifelong. A minority of cases caused by temporary thyroiditis or certain drugs may resolve. Stopping or skipping medication on your own can cause symptoms to return, so any change should be made with your clinician.
Hypothyroidism slows metabolism and can cause modest weight gain, partly from fluid retention. Treating it and normalizing hormone levels often reverses that excess, but levothyroxine is not a weight-loss drug, and using thyroid hormone for weight loss in people with normal thyroid function is unsafe. Lasting weight management still depends on diet and activity.
No diet cures hypothyroidism, and medication remains essential. Adequate (not excessive) iodine matters, but most Americans get enough; high-dose iodine or kelp can worsen autoimmune thyroid disease. People with celiac disease — which is more common in those with autoimmune thyroid disease — need a gluten-free diet for the celiac disease itself, but gluten avoidance is not an established treatment for hypothyroidism in people without celiac disease.
Some people notice improvement within a couple of weeks, but full benefit often takes one to several months as the dose is adjusted to bring TSH into range. Because levothyroxine has a long half-life and steady-state takes time, clinicians usually wait about 6–8 weeks before rechecking blood tests and changing the dose.
Subclinical hypothyroidism (high TSH, normal free T4) is often mild and may not require treatment. Decisions are individualized based on how high the TSH is, whether symptoms or thyroid antibodies are present, age, heart health, and pregnancy plans. Some people are simply monitored with periodic blood tests, while others — particularly those with markedly elevated TSH or who are pregnant — are treated.
In the United States, the most common cause is Hashimoto's thyroiditis, an autoimmune disorder in which the immune system gradually attacks and damages the thyroid gland. Worldwide, the leading cause is iodine deficiency. Other causes include thyroid surgery, radioactive iodine treatment, radiation to the neck, certain medications such as lithium and amiodarone, and, rarely, pituitary or hypothalamic disorders.
Hypothyroidism is diagnosed with blood tests, not symptoms alone, because its symptoms overlap with many conditions. The main test is TSH (thyroid-stimulating hormone), often with free T4. A high TSH with low free T4 indicates overt hypothyroidism, while a high TSH with normal free T4 suggests a milder, subclinical form. A thyroid peroxidase antibody test can confirm an autoimmune cause like Hashimoto's.
Most hypothyroidism is permanent and managed rather than cured, especially when caused by Hashimoto's disease, thyroid surgery, or radioactive iodine. Daily levothyroxine replaces the missing hormone and controls the condition effectively, but it usually must be continued for life with periodic monitoring. Some temporary forms, such as certain cases of thyroiditis, may resolve on their own. Never stop medication without consulting your clinician.
Early symptoms often develop slowly over months and can be subtle. Common signs include fatigue, unexplained weight gain, feeling cold when others are comfortable, dry skin, thinning hair, constipation, muscle aches, low mood, and trouble concentrating. Women may notice irregular or heavier menstrual periods. Because these symptoms are nonspecific, a blood test is needed to confirm whether the thyroid is the cause.
The standard treatment is levothyroxine, a synthetic thyroid hormone taken once daily, usually on an empty stomach before breakfast. The dose is tailored to each person based on weight, age, and other health factors, and is adjusted using follow-up TSH blood tests, often every 6 to 8 weeks at first. Most people feel noticeably better within weeks to a few months of starting treatment.
Untreated hypothyroidism during pregnancy can pose risks to both mother and baby, including effects on the baby's development. Thyroid hormone needs often increase during pregnancy, so the levothyroxine dose may need adjustment, and the medication is considered safe to use. Anyone who is pregnant or planning pregnancy and has thyroid concerns should work closely with their clinician to monitor and manage hormone levels.
This page is for general information and is not medical advice. Always consult a qualified clinician about diagnosis and treatment. Individual results vary.