Underactive and Overactive Thyroid

What does the thyroid do?

The thyroid is a small butterfly-shaped gland at the front of the neck. The NHS says its hormones affect heart rate and body temperature. NIDDK says that because these hormones regulate the body’s use of energy, low levels slow down many of the body’s functions.

There are two main states. In an underactive thyroid the gland does not make enough hormone. In an overactive thyroid it makes more than needed. Both can be recognized with a blood test, and both can be treated.

A common cause of an underactive thyroid is Hashimoto’s disease, in which the immune system attacks the thyroid. The NHS says it is more common in women and usually starts between ages 30 and 50. NIDDK says most cases are mild or have few obvious symptoms.

Symptoms of an underactive thyroid

The NHS says symptoms appear slowly and may be mild and easy to miss:

• Heavy tiredness
• Feeling colder than usual
• Weight gain
• Constipation
• Difficulty concentrating
• Depression or low mood
• Dry skin and hair, or hair loss
• A hoarse voice
• Irregular or heavy periods

Most of these symptoms have other causes too. For this reason, symptoms alone cannot make the diagnosis, and a test is needed.

Symptoms of an overactive thyroid

The NHS and NIDDK list these:

• Palpitations, or a fast and irregular heartbeat
• Nervousness, anxiety and irritability
• Poor sleep
• Hand tremor and muscle weakness
• Sweating and intolerance of heat
• Weight loss despite a bigger appetite
• Frequent bowel movements
• Swelling at the front of the neck (goiter)

NIDDK says that in older adults it sometimes looks like depression or forgetfulness, with loss of appetite and withdrawal.

A common cause of an overactive thyroid is Graves’ disease. The NHS says it is the cause in about four out of every five cases. Other causes are active nodules, thyroid inflammation, too much iodine, and some medicines, such as amiodarone. NIDDK points out that some cough syrups, heart medicines, seaweed and supplements contain a lot of iodine. Tell the doctor what you take.

Who should be seen?

Make an appointment if:

• You have several of the symptoms above together and for some time.
• There is thyroid disease or autoimmune disease in your family and you have a symptom.
• Your neck is swollen or your voice is hoarse and it does not get better.
• You have high cholesterol. NIDDK recommends a thyroid test in this case too.
• You are pregnant or planning pregnancy and you know you have thyroid disease.

How is it evaluated?

The doctor asks about symptoms, family history and any recent pregnancy, and feels the neck to see whether the gland is swollen. The NHS says a blood test measures thyroid-stimulating hormone (TSH) and thyroxine (T4). NIDDK adds that imaging is sometimes needed to find the cause.

To understand what a TSH number means, see the article “What Does a TSH Number Mean?”, and to prepare for the test, see the guide “Preparing for lab tests”.

Treatment of an underactive thyroid

The usual treatment is levothyroxine, a pill that replaces the hormone the thyroid is not making. NIDDK says it is a medicine like the hormone of a healthy thyroid, and if it is taken as directed, an underactive thyroid is usually fully controlled. The NHS says the doctor sets the dose separately for each person and may start with a small amount.

Monitoring is needed after starting the medicine:

• NIDDK says the blood test is usually repeated 6 to 8 weeks after starting, and again with every dose change.
• Once the dose has settled, testing is at first about every six months and then once a year.
• An incorrect dose has side effects. The NHS lists nausea, diarrhea, headache and poor sleep. NIDDK also warns that too much hormone can lead to atrial fibrillation and osteoporosis. So take neither less than needed nor more.

A few other important points:

• Take the pill every day at the same time on an empty stomach. The details are in the questions section.
• Biotin may distort the result of a thyroid test. The NHS includes it in the list of things you should tell the doctor about.
• NIDDK says foods very high in iodine, such as seaweed, and iodine supplements may make an underactive thyroid caused by Hashimoto’s worse. Ask the care team what to cut back on.
• Do not stop levothyroxine on your own. Feeling well means the medicine is working.

Treatment of an overactive thyroid

The NHS lists three main routes: antithyroid medicine, radioiodine and surgery. NIDDK explains:

• A beta blocker reduces tremor, palpitations and nervousness within hours, but it does not reduce hormone production.
• An antithyroid medicine (the most common is methimazole) reduces hormone production. It is not a permanent treatment and usually lasts one to two years. It has side effects such as rashes, a drop in white blood cells and, rarely, liver failure. If you develop tiredness or weakness, vague abdominal pain, loss of appetite, rash or easy bruising, jaundice, a persistent sore throat, or fever and chills, tell the doctor right away.
• Radioiodine gradually destroys the hormone-making cells. It is effective and common, but sometimes more than one treatment is needed, and almost everyone becomes underactive afterward and must take hormone pills.
• Surgery is done less often, for example for large goiters. If the whole gland is removed, hormone must be taken for life.

The choice of route depends on the cause, severity, age, pregnancy, heart disease and access to an experienced surgeon. The NHS says an endocrinologist usually helps with the choice. The NHS also stresses the risk of eye complications (itching, double vision, bulging eyes).

Pregnancy

NIDDK says an untreated underactive thyroid in pregnancy can affect both the mother and the baby. The NHS lists the risk of pre-eclampsia, birth defects, premature birth and miscarriage, and says treatment usually helps prevent these complications. Thyroid medicine is considered safe in pregnancy and many pregnant women need a higher amount, so contact the doctor as soon as you learn you are pregnant.

With an overactive thyroid, NIDDK says the mild form is usually not a problem but the severe form, when untreated, can affect both the mother and the baby, so if you plan to become pregnant, get the condition under control beforehand. The NHS lists the risk of pre-eclampsia, premature birth and miscarriage for an uncontrolled overactive thyroid too. NIDDK says methimazole can on rare occasions harm the fetus, so another medicine is often used in the first three months of pregnancy. Tell the doctor as soon as you know you are pregnant, and do not stop or switch the medicine yourself. Radioiodine is not used during pregnancy and breastfeeding.

When to get urgent help

• Thyroid storm: MedlinePlus calls it a rare, life-threatening condition that can develop in an untreated overactive thyroid. Signs: restlessness, confusion or a change in alertness, fever, fast heartbeat, tremor and sweating. If a person with an overactive thyroid has these signs, call 115 (Iran’s emergency number, not valid abroad).
• Myxoedema coma: The NHS says a very severe underactive thyroid rarely reaches this state, mostly in older adults, and it is a hospital emergency.
• If after taking a thyroid pill you develop severe shortness of breath or swelling of the throat or tongue, the NHS lists this as a serious allergic reaction. Call 115.

See the full list in the guide “When should I call right away?”.