Hypothyroidism vs Hyperthyroidism: Symptoms, Tests, Treatment

Hypothyroidism vs Hyperthyroidism: Symptoms, Tests, Treatment

Hypothyroidism means your thyroid makes too little hormone, so your body slows down: you may feel tired, cold, constipated and heavier. Hyperthyroidism means it makes too much, so your body speeds up: you may lose weight, feel shaky, sweat and notice a racing heart. A simple blood test for TSH is usually the first step to tell them apart, and both conditions are very treatable.

Thyroid problems are among the most common long-term hormone conditions, and they often sit alongside other common chronic diseases that build slowly. This guide compares the two conditions, explains the main causes and tests, and lists the treatments by generic name.

Key takeaways

  • Hypothyroidism (underactive thyroid) slows body functions; hyperthyroidism (overactive thyroid) speeds them up.
  • Hashimoto's disease is the most common cause of an underactive thyroid, and Graves' disease is the most common cause of an overactive one.
  • A high TSH usually points to hypothyroidism, and a low TSH usually points to hyperthyroidism.
  • Levothyroxine treats hypothyroidism; methimazole, radioactive iodine or surgery treat hyperthyroidism.
  • High fever, a heart rate over 140 and confusion in someone with an overactive thyroid can mean thyroid storm, which is an emergency.

This article is for general information and is not medical advice.

What does the thyroid actually do?

The thyroid is a small, butterfly-shaped gland at the front of your neck that sets the pace of your metabolism. Its hormones control how your body uses energy and affect nearly every organ, including how fast your heart beats, according to the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK).

The gland makes two main hormones. T4 (thyroxine) is the main hormone it releases, and T3 (triiodothyronine) is the more active form. A third hormone, TSH (thyroid-stimulating hormone), comes from the pituitary gland in your brain. TSH works like a thermostat signal: it tells the thyroid how much hormone to make.

When thyroid hormone is low, many body functions slow down. When it is too high, they run too fast. That is why the two conditions feel almost like opposites.

How do the symptoms of an underactive and overactive thyroid compare?

Most symptoms are mirror images: an underactive thyroid causes slowing, while an overactive thyroid causes speeding up. The table below puts them side by side using information from the NIDDK pages on hyperthyroidism and hypothyroidism.

FeatureHypothyroidism (underactive)Hyperthyroidism (overactive)
Hormone levelToo little thyroid hormoneToo much thyroid hormone
EnergyTiredness, slowed thinkingNervousness, irritability, trouble sleeping, fatigue
WeightWeight gainWeight loss despite a bigger appetite
TemperatureFeeling coldSweating, trouble tolerating heat
HeartSlower heart rateFast or irregular heartbeat
DigestionConstipationMore frequent bowel movements
Skin, hair, musclesDry skin, thinning hair, muscle achesShaky hands, muscle weakness
Most common causeHashimoto's diseaseGraves' disease
How common (US, age 12+)Nearly 5 in 100 peopleAbout 1 in 100 people
Main treatmentLevothyroxineMethimazole, radioactive iodine or surgery

Both conditions can cause a goiter, which is a visibly enlarged thyroid at the front of the neck. Both are also more common in women and in people over 60, according to NIDDK.

Symptoms can be vague. Low mood and poor memory from an underactive thyroid can look a lot like depression symptoms, so it is reasonable to ask whether a thyroid test makes sense if you are being checked for low mood. In older adults, NIDDK notes that an overactive thyroid can be mistaken for depression or even dementia, and may show up as loss of appetite or withdrawal rather than the classic racing heart.

What causes hypothyroidism and hyperthyroidism?

Autoimmune disease causes most cases of both: Hashimoto's disease damages the thyroid so it makes too little hormone, and Graves' disease overstimulates it so it makes too much. An autoimmune disease is one where your immune system attacks your own tissue by mistake.

Hashimoto's disease

In Hashimoto's disease, the immune system attacks the thyroid and slowly damages it. NIDDK says it is 4 to 10 times more common in women than in men and most often starts between ages 30 and 50. Your risk is higher if relatives have it or if you have another autoimmune condition, such as type 1 diabetes, celiac disease or lupus. If you have Hashimoto's alongside digestive issues, our guide to gentle gluten-free meals for gastroparesis and Hashimoto's covers practical eating tips.

Other causes of an underactive thyroid include thyroid surgery, radioactive iodine treatment, radiation to the neck or chest, and pregnancy within the past 6 months, according to NIDDK.

Graves' disease

Graves' disease is the most common cause of an overactive thyroid. Here the immune system makes an antibody that pushes the thyroid to produce too much hormone. Other causes listed by NIDDK include:

  • Toxic thyroid nodules: lumps, usually not cancer, that make extra hormone, most often in older adults.
  • Thyroiditis: inflammation of the thyroid that leaks stored hormone into the blood, including after giving birth.
  • Too much iodine: from some cough syrups, some heart medicines, seaweed or seaweed supplements.
  • Too much thyroid hormone medicine in people being treated for hypothyroidism.

Autoimmune conditions tend to cluster. If you already manage one, such as type 1 diabetes (see type 1 vs type 2 diabetes), your doctor may watch your thyroid more closely.

Which tests diagnose a thyroid problem?

A TSH blood test is usually the first test, and T4, T3 or antibody tests follow if the result is abnormal. According to MedlinePlus, a high TSH means the pituitary is pushing a sluggish thyroid to work harder, which suggests hypothyroidism. A low TSH means the pituitary has backed off because hormone levels are already too high, which suggests hyperthyroidism.

A typical testing path looks like this:

  1. TSH test: shows whether a thyroid problem may be present, but not what is causing it.
  2. Free T4 (and sometimes T3) test: measures the hormone itself to confirm whether levels are low or high.
  3. Thyroid antibody tests: look for autoimmune disease. NIDDK says thyroid peroxidase (TPO) antibodies are present in most people with Hashimoto's.
  4. Imaging if needed: a thyroid ultrasound can help when antibodies are negative but Hashimoto's is still suspected, or when a lump is found.

A few things can shift results. MedlinePlus notes that serious illness can briefly lower TSH, TSH can run higher in people over 80 without thyroid disease, and TSH is often slightly low in the first trimester of pregnancy. Tell your clinician about every medicine and supplement you take, and do not stop any medicine before a test unless they tell you to.

How is each condition treated?

Hypothyroidism is treated by replacing the missing hormone with levothyroxine, while hyperthyroidism is treated by lowering hormone production with methimazole, radioactive iodine or surgery.

Treating an underactive thyroid

Levothyroxine is a lab-made version of T4, the hormone a healthy thyroid makes. NIDDK says it comes as a pill, liquid or soft gel and is often taken in the morning before eating. You will usually have a blood test about 6 to 8 weeks after starting, with the dose adjusted and rechecked after each change. Once your dose is stable, testing is usually repeated at 6 months and then once a year.

Most people control the condition fully with medicine, but you should never stop it without talking to your doctor. Too much hormone over time can raise the risk of atrial fibrillation (an irregular heart rhythm) and osteoporosis, so the dose matters.

Treating an overactive thyroid

  • Beta-blockers: ease tremor, nervousness and a fast heartbeat quickly, but do not lower hormone production.
  • Antithyroid medicines: methimazole is used most often; propylthiouracil is often preferred in the first trimester of pregnancy. They lower hormone production but are not a permanent cure.
  • Radioactive iodine (iodine-131): slowly destroys hormone-making thyroid cells. It is not used during pregnancy or breastfeeding, and almost everyone who has it later develops hypothyroidism, which is then treated with daily levothyroxine.
  • Surgery: removes part or most of the thyroid. If the whole gland is removed, you need lifelong hormone replacement.

Thyroid hormone also affects the heart and cholesterol, so it helps to know your other numbers. Our guide on what blood pressure numbers mean is a useful companion, since NIDDK advises that people with high cholesterol be tested for hypothyroidism.

Who should be screened for thyroid disease?

Routine screening of healthy adults with no symptoms is not clearly supported, but testing makes sense if you have symptoms or risk factors. The US Preventive Services Task Force concluded in 2015 that there is not enough evidence to weigh the benefits and harms of screening nonpregnant adults without symptoms. That statement does not apply to pregnant people or anyone with symptoms.

Based on NIDDK guidance, ask your clinician about a thyroid test if you:

  • Have symptoms from the table above that have lasted weeks.
  • Are a woman, are over 60, or have a family history of thyroid disease.
  • Have had thyroid surgery, radioactive iodine, or radiation to the neck or chest.
  • Have type 1 or type 2 diabetes, celiac disease, rheumatoid arthritis, lupus, Sjogren's syndrome or pernicious anemia.
  • Have high cholesterol, are having trouble getting pregnant, or were pregnant in the past 6 months.

Most newborns in the US are also tested at birth, because untreated congenital hypothyroidism can cause growth failure and intellectual disability.

When to see a doctor

Book a routine appointment if you notice ongoing fatigue, unexplained weight change, a swelling in your neck, a fast or irregular heartbeat at rest, or changes in your periods. Bring a list of your symptoms and any supplements, especially ones containing iodine or kelp.

Go to an emergency department right away if you or someone with a known or suspected overactive thyroid has signs of thyroid storm, a sudden and severe worsening of hyperthyroidism. Cleveland Clinic lists these red flags:

  • High fever, often 104 to 106 degrees F.
  • Heart rate above 140 beats per minute.
  • Agitation, confusion or delirium.
  • Signs of heart failure, such as severe breathlessness or swelling.
  • Fainting or loss of consciousness.
  • Severe vomiting, diarrhea, belly pain or yellowing of the skin or eyes.

Thyroid storm often follows a trigger such as stopping antithyroid medicine suddenly, surgery, infection, childbirth or a large iodine load, for example from iodinated contrast dye used in scans. If you are booked for a scan, read about contrast dye risks and prevention and tell the team about your thyroid. Cleveland Clinic reports that roughly 10% to 30% of thyroid storm cases are fatal even with treatment, so do not wait it out.

Common mistakes to avoid

  • Stopping medicine on your own. Quitting levothyroxine brings symptoms back, and stopping antithyroid medicine suddenly can trigger thyroid storm.
  • Taking iodine or kelp supplements "for the thyroid." NIDDK warns that large amounts of iodine can worsen both hypothyroidism and hyperthyroidism.
  • Judging your dose by how you feel alone. Follow the 6 to 8 week blood test schedule after any dose change.
  • Assuming fatigue or low mood is just stress. A simple TSH test can rule a thyroid cause in or out.
  • Forgetting to mention thyroid disease before surgery or a contrast scan. Both are known triggers for thyroid storm in people with an overactive thyroid.

FAQ

Can hypothyroidism turn into hyperthyroidism?

It is uncommon, but it can happen the other way around. Almost everyone treated with radioactive iodine for an overactive thyroid later becomes underactive. Hashimoto's disease rarely causes a short overactive phase before hormone levels fall.

Is a normal TSH enough to rule out a thyroid problem?

For most people, a normal TSH makes a thyroid problem unlikely. Your clinician may still check T4 if symptoms persist or you are pregnant, since TSH can run slightly low in early pregnancy and, rarely, reflects a pituitary problem.

Do I have to take thyroid medicine for life?

Most people with Hashimoto's or a removed thyroid need levothyroxine long term. Antithyroid medicines for Graves' disease are often used for a period of time rather than for life, and some people later need other treatment.

Can diet cure a thyroid condition?

No diet cures hypothyroidism or hyperthyroidism. Eating well supports your general health, but the main thing to avoid is very high iodine intake from seaweed or supplements unless your clinician advises it.

What to do next

  1. Write down your symptoms, how long they have lasted and any family history of thyroid or autoimmune disease.
  2. Ask your clinician whether a TSH test, and if needed a free T4 and antibody test, is right for you.
  3. If you start treatment, put your 6 to 8 week follow-up blood test in your calendar.
  4. Review your supplements and cough medicines for hidden iodine with a pharmacist.
  5. Save the thyroid storm red flags above so you can act fast if they appear.

Sources

Last reviewed: November 2026.

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