Hypothyroidism

Introduction

Feeling sluggish and tired? Are your hair and skin more dry and coarse than normal? If so, you may have a malfunctioning thyroid gland - specifically, a condition known as hypothyroidism or low thyroid hormone levels. Hypothyroidism is a syndrome that affects approximately 1.5% to 2% of women and 0.2% of men. The incidence increases as age increases. While these percentages may seem small, hypothyroidism is actually the second most common disease of a hormone-producing gland, coming in second only after diabetes. Read on to learn more about this condition.

What is it?

Hypothyroidism is a medical condition in which the thyroid gland does not produce enough thyroid hormone. Most often it results when part of the thyroid gland fails to work properly. This is referred to as "primary hypothyroidism." It accounts for 95% of all the cases of hypothyroidism. However, hypothyroidism can also be due to a failure of the pituitary gland - the organ responsible for telling the thyroid gland to produce thyroid hormone. When a malfunctioning pituitary gland causes low thyroid hormone levels, the condition is called "secondary hypothyroidism."

So what is the thyroid gland and why is thyroid hormone important? The thyroid gland is a small, butterfly-shaped gland located in the neck just under the Adam's apple. The thyroid gland produces thyroid hormone, which acts on many different parts of the body including the muscles, brain, heart, intestines, and skin. Thyroid hormone helps regulate many bodily functions such as water balance, fat and sugar metabolism, and body temperature. A decrease in thyroid hormone can affect the proper functioning of one or many of these parts of the body.

What causes it?

Numerous factors cause hypothyroidism. These factors are grouped as causes of primary hypothyroidism and causes of secondary hypothyroidism. Each of the factors is discussed below.

Primary hypothyroidism

  1. Hashimoto's Disease

    2. Hashimoto's disease occurs when the immune or protective mechanisms of the body mistakenly attack the thyroid gland. When the immune system attacks the thyroid gland, it damages part of the gland in such a way that it can no longer produce thyroid hormone.

  2. Iatrogenic hypothyroidism

    3. Iatrogenic hypothyroidism occurs when the thyroid gland has been exposed to large amounts of radiation. The radiation kills certain cells in the thyroid gland that are responsible for producing thyroid hormone. This occurs most often in patients who have received radiation treatment for Graves' disease or neck cancers.

  3. Consumption of high iodine containing foods and medicines (goitrogens)

    4. Excess consumption of iodized salt, seaweed, cabbage, brussel sprouts, broccoli, cauliflower, mustard, turnips, canola, oil, soy, pine and peanuts. Also, iodides (SSKI, Lugol's solution), which are often used in combination with surgery to treat hyperthyroidism can cause hypothyroidism if not used properly.

  4. Iodine deficiency

    5. The thyroid gland requires iodine as one of its building blocks for making thyroid hormone. If there is a lack of iodine in the body, the thyroid gland will not have enough to make the amount of thyroid hormone needed for the body to function properly.

  5. Enzyme defects

    6. The thyroid gland also requires certain enzymes to make thyroid hormone. Enzymes are proteins that the body uses to help convert one substance into another.

  6. Thyroid gland removal

    7. If cancer is present in the thyroid gland, part or all of the thyroid gland often needs to be removed. With the removal of part or all of the thyroid gland, less thyroid hormone is produced.

  7. Medication

    8. Certain medications (such as lithium, interferon, and amiodarone) can have a negative effect on the thyroid gland causing it to produce less thyroid hormone.

  8. Other diseases

    9. Certain diseases that affect the connective tissues in the body (for example, sarcoidosis, amyloidosis, and scleroderma) can affect the thyroid gland as well. When these diseases reach the thyroid gland, they add tissue (called connective tissue) that is different than thyroid gland tissue, which ends up taking the place of the thyroid gland tissue. The connective tissue that replaces the thyroid gland tissue does not have the ability to produce thyroid hormone.

  9. Congenital disease

    10. Although it is rare, approximately 1 in 5,000 babies are born with a malfunctioning thyroid gland or no thyroid gland at all.

Secondary hypothyroidism

  1. Pituitary disease

    2. Pituitary disease can be caused by tumors, surgery, or radiation therapy. Each of these causes a decrease in pituitary gland size and functioning. When the pituitary gland gets smaller as the result of tumors, surgery, or radiation, it cannot produce enough hormone (known as thyroid stimulating hormone--or TSH). TSH tells the thyroid gland to produce thyroid hormone.

  2. Sheehan's syndrome

    3. Sheehan's syndrome occurs when there is necrosis (cell death) of the pituitary gland. This syndrome is related to severe bleeding during childbirth in women.

  3. Hypothalamic hypothyroidism

    4. Just as the pituitary gland releases thyroid stimulating hormone (TSH) to tell the thyroid gland to produce thyroid hormone, there is a hormone (called thyrotropin-releasing hormone) released from the hypothalamus, an area in the brain that tells the pituitary gland to release parathyroid hormone. Although it is rare, there is sometimes a problem with the hypothalamus in which thyrotropin-releasing hormone is not released. This can be caused by radiation to the brain, trauma to the head, or other conditions that affect the brain.

Who has it?

Hypothyroidism affects an estimated 10 million Americans. Eight million of these Americans don't even know they have it and have never been treated. The condition occurs most commonly in women (approximately 2 out of 100 women) and mainly in women over the age of 40 years. Although it is much less common, hypothyroidism does occur in approximately 2 out of 1000 males. It is important to note that hypothyroidism is more likely to occur as age increases. In the general population, as many as 18 out of 1000 people have hypothyroidism.

What are the risk factors?

There are a few risk factors that are known to be associated with the development of hypothyroidism. The most common risk factors are listed below.

  • Radiation therapy aimed at the thyroid, neck, or throat

  • Surgical removal of part or all of the thyroid gland

  • Taking certain medications (mainly lithium, amiodarone, propylthiouracil and methimazole)

  • A family history of autoimmune disease or thyroid disorders

  • Having diabetes

  • Being a woman over the age of 40 years

  • Having iodine deficiency

What are the symptoms?

The symptoms of hypothyroidism may vary widely. Each of the symptoms listed below is seen in 60% or more of the patients with hypothyroidism.

  • Weakness or fatigue
  • Dry skin
  • Abnormal drowsiness
  • Slowed speech or a hoarse voice
  • Eyelid swelling
  • Cold intolerance
  • Unexplained weight gain
  • Muscle cramps and stiffness
  • Thickened tongue
  • Facial swelling
  • Coarse hair and skin
  • Pale skin
  • Forgetfulness
  • Constipation

In older patients, hypothyroidism can sometimes be confused with Alzheimer's disease because patients with hypothyroidism can also have memory problems and impaired cognitive function. Patients with hypothyroidism may also be depressed. Women who are pregnant and develop hypothyroidism may become depressed, and the condition may be confused with postpartum depression. Also, hypothyroidism can cause elevated cholesterol, which can then lead to heart disease.

How is it treated?

The goals of treatment for hypothyroidism include restoration of normal thyroid hormone levels, relief of symptoms, and reversal of any metabolic abnormalities that were associated with the low thyroid hormone levels, such as decreased sugar and fat metabolism or body temperature regulation. The main treatment for hypothyroidism is thyroid hormone replacement therapy.

Since the thyroid gland is not producing enough thyroid hormone, the hormone must be supplemented or replaced by daily use of synthetic or natural thyroid hormone. Natural thyroid hormone is derived from pig, cow, or sheep thyroid glands, whereas synthetic thyroid hormone is created manually in a laboratory. Synthetic thyroid hormone - mainly a drug called levothyroxine (various brands and generic formulations, e.g., Levo-T, Levoxyl, Synthroid, Unithroid) - is usually the drug of choice for hypothyroidism because it is relatively inexpensive, causes practically no side effects when dosed properly, is chemically stable, and has uniform potency. However, all thyroid hormone replacement therapies - whether natural or synthetic - have the ability to effectively restore thyroid hormone levels and can be used to treat hypothyroidism. Generally, once you start a thyroid hormone replacement therapy, you should stick with it and not change to another thyroid hormone replacement therapy unless under the direction of your doctor.

Most thyroid hormone replacement therapies come in tablet form, although an injectable form is also available. The best time to take thyroid hormone replacement therapy is in the morning. However, it needs to be taken at least 30 minutes before eating because some fiber and bran products can decrease the amount of thyroid hormone that gets absorbed from the stomach into the bloodstream. Other medications can also affect the amount of thyroid hormone absorbed into the bloodstream. Thyroid hormone replacement therapy should be separated from the following other medications by at least 2 hours: iron, antacids, sucralfate, cholestyramine.

There is no cure for hypothyroidism. This means that a patient with hypothyroidism will most likely have to take daily thyroid hormone replacement therapy for the rest of his or her life. In patients with hypothyroidism who are taking thyroid hormone replacement therapy, thyroid hormone levels should be measured by a doctor at least every 6 months, with adjustments made to the thyroid hormone replacement therapy as needed. Patients should alert their doctor to any symptoms that may suggest high or low thyroid hormone levels - this may mean the dose of the thyroid hormone replacement therapy needs adjustment. Symptoms that might indicate low levels are listed in the "What are the symptoms?" section. Symptoms of high thyroid hormone levels include nervousness, fast heart rate, shortness of breath, warm and moist skin, and unexplained weight loss.

To learn more about the specific drugs used as thyroid hormone replacement therapy, click on the Drug Class links below.

What is on the horizon?

Researchers are currently looking for a genetic cause of hypothyroidism. If a genetic cause is found, they may be able to develop better medicines to treat or prevent - or even cure - hypothyroidism.

References

  1. Woeber, KA. Update on the management of hyperthyroidism and hypothyroidism. Archives of Family Medicine. 2000; 9: 743-747.

  2. Hueston, WJ. Treatment of hypothyroidism. American Family Physician. 2001; 64: 1717-1724.

  3. Reasner CA, Talbert RL. Thyroid disorders. In: DiPiro JT, Talbert RL, Yee GC, Matzke GR, Wells BG, Posey LM, editors. Pharmacotherapy: A Pathophysiologic Approach. 6th ed. New York: McGraw-Hill; 2005: 1369-1390.

  4. The American Thyroid Association. Available at: http://www.thyroid.org/ Accessed July 31, 2007 and August 18, 2008.

  5. Hypothyroidism. Mayo Clinic.com. Available at: http://www.mayoclinic.com/invoke.cfm?id=DS00353. Accessed August 15, 2007 and August 18, 2008.

  6. National Institutes of Health. Clinical Trials. Available at: http://www.clinicaltrials.gov/. Accessed July 31, 2007 and August 18, 2008.

  7. General Practice Notebook: Goitrogens. Available at: http://www.gpnotebook.co.uk/cache/-1227882489.htm. Accessed July 31,2007.

    Hypothyroidism Health Condition Last Updated: August 2008

    Note: The above information is intended to supplement, not substitute for, the expertise and judgment of your physician, pharmacist, or other healthcare professional. It is not intended to diagnose a health condition, but it can be used as a guide to help you decide if you should seek professional treatment or to help you learn more about your condition once it has been diagnosed.

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