Hypothyroidism is an underactive thyroid gland ("hypo-" means "under" or "below normal").  Hypothyroidism means that the thyroid gland cannot make enough thyroid hormone to keep the body running normally.  People are hypothyroid if they have too little thyroid hormone in their blood.  Common causes are autoimmune disease, surgical removal of the thyroid, and radiation treatment.  Low thyroid hormone levels cause the body’s functions to slow down, leading to general symptoms like dry skin, fatigue, loss of energy, and memory problems.  Hypothyroidism is treated by replacing the missing thyroid hormone with synthetic thyroxine pills, which the person must take every day for life.  With daily treatment, most patients recover completely. 

Symptoms

When thyroid hormone levels are too low, the body's cells can't get enough thyroid hormone and the body's processes start slowing down.  For example, the body makes less heat and less energy, and organs like the brain and bowels move more slowly.  As the body slows, you may notice that you feel colder, you tire more easily, your skin is getting drier, you're becoming forgetful and depressed, and you've started getting constipated.  
 
You may also have body changes that you can’t feel.  For example, you may not know that cholesterol is building up in your blood and "plaque" is hardening your arteries, increasing your risk for heart attack.  So hypothyroidism doesn't just cause symptoms.  It can make other conditions worse.  

Some people develop hypothyroidism quickly over a few months.   Some people develop it slowly over many years.  In general, the lower thyroid hormone levels become and the longer they stay low, the more severe the symptoms will be.  Subclinical (mild) hypothyroidism may cause mild symptoms or none at all.  Severe hypothyroidism usually causes severe symptoms.  But no one can predict exactly which symptoms a person will develop or how severe they'll be.  Some people are very sick by the time they learn their diagnosis; others whose blood tests show severe hypothyroidism have few if any symptoms. 

  Because the symptoms are so variable, the only way to know for sure whether you have hypothyroidism is with blood tests.

Common features of hypothyroidism

Less energy 

More fatigue, trouble awakening in the morning, need for more sleep, and tendency to fall asleep during the day 

Feeling cold when other people feel warm 

Less sweating 

Drier, itchier skin 

Yellow or orange skin, caused by a build-up of the pigment carotene from fruits and vegetables 

Drier, coarser, more brittle hair 

More hair loss (the amount differs in different people; patients don’t go bald but their hair can look thin) 

Loss of appetite 

Mild weight gain (5-20 pounds) and difficulty losing weight (hypothyroidism doesn’t cause obesity) 

New or worsening problems with memory, slower thinking 

New snoring 

More frequent and severe muscle cramps and joint aches 

New feeling of pins and needles in the hands and feet (paresthesia) 

New constipation 

New puffiness around the face (especially the eyes), hands, ankles, and feet because of fluid build-up 

Carpal tunnel syndrome 

Heavier and/or more frequent menstrual periods, worse cramps, worse premenstrual symptoms 

Feeling irritable 

New depression, sadness or not caring about anything 

New hoarse voice 

New hearing loss 

Goiter (swelling in the front of the neck, caused by enlargement of the thyroid; goiter is most likely to be part of Hashimoto's thyroiditis) 

Shrinking thyroid (most likely in atrophic thyroiditis) 

Slowing of heart rate, reducing the blood flow and oxygen delivered to body tissues 

Slightly higher blood pressure, caused by stiffening of arteries 

Higher cholesterol levels 

Causes 

There can be many reasons why the cells in the thyroid gland can’t make enough thyroid hormone.  Here are the major causes, from the most to least common: 

Autoimmune disease

The immune system normally protects the body against bacterial and viral .invaders.  In autoimmune diseases (.auto. means .self.), the immune system mistakenly attacks a normal part of the body.  In autoimmune hypothyroidism, the immune system accidentally attacks cells in the thyroid gland, interfering with their ability to make thyroid hormone.  When enough thyroid cells have been destroyed, too few are left to meet the body’s need for thyroid hormone.  Autoimmune thyroid disease is more common in women than men.  It can start at any age, but becomes more common as people get older.  In women, it often begins during pregnancy, after delivery, or around menopause.  The cause is probably a combination of an inherited tendency and an as yet unknown trigger.  No one knows whether stress plays a role.  Autoimmune thyroiditis can begin suddenly, but in most patients it develops slowly over years.  The most common forms are Hashimoto’s thyroiditis and atrophic thyroiditis. 

Surgical removal of part or all of the thyroid gland

Some people with thyroid nodules, thyroid cancer, or Graves' disease need to have part or their entire thyroid removed.  If the whole thyroid is removed, people will definitely become hypothyroid.  If only part of the thyroid is removed, the remaining part may: 

. Work well enough to keep thyroid hormone blood levels normal throughout the person’s life 

. Never make enough thyroid hormone to keep blood levels normal 

. At first make enough thyroid hormone, but later become unable to meet the body’s need for hormone 

Radiation treatment

Some people with Graves' disease, nodular goiter, or thyroid cancer are given radioactive iodine (I131) to destroy their thyroid gland purposely.  Patients with Hodgkin’s disease, lymphoma, or cancers of the head or neck are treated with radiation.  All these patients can lose part or all of their thyroid function.  

Congenital hypothyroidism (hypothyroidism that a baby is born with) 

A few babies are born without a thyroid or with a partly formed thyroid.  A few babies have part or their entire thyroid in the wrong place (ectopic thyroid).  In some babies, the thyroid cells or their enzymes don’t work right.  Babies with any of these problems may be hypothyroid from birth.  In some, the thyroid may make enough hormones for a while and then may no longer be able to keep up with the need, so the person becomes hypothyroid as an older child or even as an adult.  

Thyroiditis

Thyroiditis is an inflammation of the thyroid gland, usually caused by an autoimmune attack (in postpartum thyroiditis or silent thyroiditis) or by a viral infection.  Thyroiditis can make the thyroid release its entire supply of stored thyroid hormone into the blood at once, causing brief Hyperthyroidism (an overactive thyroid); once the entire stored hormone has been released, the thyroid becomes under active.  Almost all patients with viral thyroiditis recover their thyroid function, but about one-fourth of patients with autoimmune thyroiditis have permanent hypothyroidism. 

Medicines

Medicines like amiodarone, lithium, interferon alpha, and interleukin-2 can interfere with the thyroid gland’s ability to make thyroid hormone.  These drugs are most likely to trigger hypothyroidism in patients who have a genetic tendency to autoimmune thyroid disease.  It’s also possible that treatment with thalidomide for multiple myeloma can cause hypothyroidism. 

Too little or too much iodine

The thyroid gland must have iodine to make thyroid hormone.  Iodine comes into the body in foods, mainly dairy products, chicken, beef, pork, fish, and iodized salt.  The iodine then travels through the blood to the thyroid.  Keeping thyroid hormone production in balance requires the right amount of iodine.  People who live in undeveloped parts of the world may not get enough iodine in their diet.  Worldwide, iodine deficiency is the most common cause of hypothyroidism, although it is a rare cause in the U.S.  Taking in too much iodine can also cause or worsen hypothyroidism.  The major source of too much iodine is dietary supplements containing kelp, a kind of seaweed.  Most such supplements are sold with the false promise of helping people lose weight.  Other sources of too much iodine are x-ray dyes, medicines like amiodarone, and some older expectorants (medicines that help clear the lungs and throat). 

Damage to the pituitary gland

The pituitary tells the thyroid how much hormone to make.  If the pituitary is damaged by a tumor, radiation, or surgery, it may no longer be able to give the thyroid the right instructions, and the thyroid may stop making enough hormones. 

Rare disorders that infiltrate the thyroid

In a few people, diseases deposit abnormal substances in the thyroid.  For example, amyloidosis can deposit amyloid protein, sarcoidosis can deposit granulomas, and hemochromatosis can deposit iron.  These deposits can prevent the thyroid from working right.  

What does not cause hypothyroidism?

. Soy does not cause hypothyroidism, but it does interfere with the body’s ability to absorb thyroid replacement therapy. 

. The amounts of broccoli, cabbage, and Brussels sprouts that people eat in a normal diet do not cause hypothyroidism. 

. There is no evidence that some people’s thyroid gland makes enough hormones but it does not get to the body’s cells. 

Who is at risk?

Hypothyroidism is one of the most common thyroid disorders.  It affects people all over the world, of every age, sex, race, and level of wealth and education.  About 2-3% of Americans have pronounced hypothyroidism, and as many as 10-15% have subclinical (mild) hypothyroidism.  More than half of people who have hypothyroidism don’t know it.  

The most common cause of hypothyroidism is autoimmune disease.  Risk factors for autoimmune hypothyroidism are: 

. Family history:  Having a relative with autoimmune thyroid disease 

. Older age:  Hypothyroidism can start at any age, but the risk keeps growing as people get older 

. Being female:  Hypothyroidism is more common in women than men.  It is much more common in young women than young men, but as men get older, they start to catch up 

. Being white or Asian:  Hypothyroidism is common in whites and Asians.  African-Americans are at lower risk 

. Having another autoimmune disorder, such as type 1 diabetes, rheumatoid arthritis, multiple sclerosis, celiac disease, Addison’s disease, pernicious anemia, vitiligo, or even prematurely gray hair 

. Having Down syndrome or Turner’s syndrome 

. Having bipolar disease (manic-depression)  

The rate of hypothyroidism goes up: 

. During pregnancy 

. After delivery 

. Around menopause