Hypothyroidism (Underactive Thyroid)

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.

What is hypothyroidism (underactive thyroid)?

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:

• Primary hypothyroidism — the thyroid gland itself underperforms. This is by far the most common form and is coded E03.9 (hypothyroidism, unspecified) or E06.3 (autoimmune thyroiditis) in ICD-10.
• Central (secondary) hypothyroidism — the pituitary or hypothalamus fails to signal the thyroid properly. This is uncommon.
• Subclinical hypothyroidism — a milder, often early state defined by a high TSH with a *normal* free T4 (StatPearls).

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.

What causes hypothyroidism and who is at risk?

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:

• Thyroid surgery — removing part or all of the thyroid (for nodules, cancer, or other reasons).
• Radioactive iodine or radiation therapy — often used to treat hyperthyroidism (overactive thyroid) or head and neck cancers, which can leave the gland underactive.
• Certain medications — including lithium, amiodarone, and some cancer immunotherapies and tyrosine kinase inhibitors (NIDDK).
• Congenital hypothyroidism — present from birth; in the U.S. all newborns are screened for it.
• Pituitary or hypothalamic disease — the cause of central hypothyroidism.
• Thyroiditis — temporary inflammation, including after pregnancy (postpartum thyroiditis), that can cause a transient underactive phase.

Risk factors that raise the likelihood of developing hypothyroidism include (NIDDK):

• Being a woman, especially over age 60
• A personal or family history of thyroid or other autoimmune disease (such as type 1 diabetes, celiac disease, or rheumatoid arthritis)
• Prior thyroid surgery or radioactive iodine treatment
• Being pregnant or having delivered within the past six months
• Down syndrome or Turner syndrome
• Iodine intake that is too low — or, in people with autoimmune thyroid disease, too high

What are the symptoms of hypothyroidism?

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):

• Fatigue, sluggishness, and low energy
• Unexplained weight gain or difficulty losing weight
• Feeling cold when others are comfortable (cold intolerance)
• Dry skin, brittle nails, and thinning or coarse hair
• Constipation
• Puffiness of the face and swelling, especially around the eyes
• Muscle aches, weakness, and joint stiffness
• Slowed heart rate
• Heavier or irregular menstrual periods
• Depressed mood, "brain fog," and trouble concentrating or remembering
• A hoarse voice or an enlarged thyroid (goiter)

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.

How is hypothyroidism diagnosed?

Hypothyroidism is diagnosed with blood tests, not symptoms alone (NIDDK).

Tests and thresholds

• TSH (thyroid-stimulating hormone) is the primary screening test. A high TSH suggests the thyroid is underactive. Laboratory reference ranges vary, but TSH is commonly considered normal at roughly 0.4 to 4.0–4.5 mIU/L; the upper limit can be adjusted for age and pregnancy (StatPearls).
• Free T4 is checked when TSH is abnormal. A high TSH with a low free T4 confirms overt (clinical) primary hypothyroidism.
• High TSH with a normal free T4 defines subclinical hypothyroidism (StatPearls).
• Thyroid peroxidase (TPO) antibodies are often measured to identify autoimmune (Hashimoto's) thyroiditis, the most common underlying cause.

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.

How is hypothyroidism treated?

Hypothyroidism is highly treatable. Treatment aims to restore normal thyroid hormone levels, relieve symptoms, and bring TSH back into the target range.

Lifestyle and supportive measures

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.

Over-the-counter options

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).

Prescription medications

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:

• How it's taken: typically on an empty stomach, about 30–60 minutes before breakfast (or several hours after the last meal), with water, to maximize absorption (StatPearls).
• Dosing and monitoring: the dose is individualized by weight, age, cause, cardiac risk, and pregnancy status, then fine-tuned. TSH is generally rechecked about 6–8 weeks after starting or changing the dose, and the dose is adjusted until TSH is in range. Once stable, monitoring usually shifts to roughly once a year (StatPearls).
• Interactions to watch: absorption can be reduced by iron and calcium supplements, antacids, proton pump inhibitors, sucralfate, and some other drugs and foods; these should be separated by several hours (StatPearls; PMID 25040647). Switching brands or formulations can also affect levels.

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).

Can hypothyroidism be prevented or managed long-term?

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:

• Taking medication consistently, the same way each day, and not stopping without medical advice
• Keeping regular follow-up and blood tests so the dose stays correct as weight, age, pregnancy, or other medications change
• Telling all prescribers about thyroid medication, since new drugs and supplements can shift the dose
• Reporting symptoms of under- or over-treatment promptly

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).

When should you see a doctor?

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):

• Profound drowsiness, confusion, or unresponsiveness
• Very low body temperature (hypothermia)
• Markedly slow heart rate, low blood pressure, or slow, shallow breathing
• Severe swelling of the face, hands, or legs

Myxedema coma requires immediate hospital treatment (StatPearls). It is uncommon but underscores why hypothyroidism should not be left untreated.

What is the outlook (prognosis) for hypothyroidism?

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.

Frequently asked questions about hypothyroidism

Will I have to take thyroid medication forever?

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.

Can hypothyroidism cause weight gain, and will medication make me lose it?

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.

Does diet, including iodine or gluten, treat hypothyroidism?

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.

How soon will I feel better after starting treatment?

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.

Is subclinical hypothyroidism dangerous, and should it be treated?

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.