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What Is Hypothyroidism?

Sunday, April 21, 2013 - 17:44
Contributors to this article: 

David Cooper MD
Guy Slowik FRCS


Hypothyroidism is a condition in which there is too little thyroid hormone in the bloodstream. The thyroid gland, which produces the thyroid hormones, is said to be "underactive," because it does not produce enough thyroid hormone for the body to function normally.

Inadequate production of thyroid hormone affects stimulation of cells and organs in the body. The low level of thyroid hormone causes the symptoms associated with hypothyroidism, which is generally a "slowing-down" of the body's processes. These symptoms include slowed heart rate, tiredness, inability to tolerate cold, mental fatigue, and constipation.

Hypothyroidism is a common condition, and it can be successfully treated. However, the symptoms are often subtle. People may believe their symptoms are due to stress, depression, or "getting older." Frequently, they may mistake the signs and symptoms of hypothyroidism for other conditions. It is not unusual for someone with hypothyroidism to go undiagnosed, sometimes for many years. Some experts estimate that as many as 9 million people in the U.S. have undiagnosed hypothyroidism.

Low thyroid levels can affect both men and women, but it is far more common in women.


Need To Know:

A different thyroid-related condition involves too much thyroid hormone in the bloodstream, producing symptoms that include rapid heart beat, extreme fatigue, weight loss, and nervousness. This condition is called hyperthyroidism, and the thyroid gland is said to be "overactive."

For further information about hyperthyroidism, go to Hyperthyroidism.


Nice To Know:

What is the thyroid gland?

The thyroid gland is a small, butterfly-shaped gland that lies just under the Adam's apple in the neck. There are two lobes to the gland, and they lie just in front and at either side of the windpipe (trachea). The thyroid is part of the body's endocrine system, which consists of glands that secrete hormones into the bloodstream.

The thyroid gland secretes thyroid hormones, which control the speed at which the body's chemical functions proceed (metabolism). To produce thyroid hormones, the thyroid gland needs iodine, an element contained in many foods. The thyroid gland also produces a hormone, called calcitonin, which may be involved in the metabolism of bones.

What Is Thyroid Hormone?

Hormones are chemical messengers released into the bloodstream by specialized glands called endocrine glands. A hormone circulates through the body in the bloodstream, delivering messages to other parts of the body. The "message" causes effects far from the gland that produced the hormone.

Thyroid hormone is produced in the thyroid gland, which is located in the front of the neck. It is released by the thyroid gland into the bloodstream and circulates throughout the body. Almost every cell in the body, from those in the brain to those in the feet, responds to the hormone.

There are two different forms of thyroid hormone present in the bloodstream. The two forms of thyroid hormone differ in the number of iodine units or atoms attached to the hormone. Iodine is a very important component of thyroid hormone.

  • Thyroid hormone with four iodine units is abbreviated as T4.
  • Thyroid hormone with three iodine units is abbreviated as T3.
  • Most thyroid hormone in the blood is T4.
  • T3 is the form that is active in the body. T4 is inactive.
  • Certain cells in the body convert T4 to T3.

Nice To Know:

Just about all the iodine we consume in food is used by the body for the production of thyroid hormone.

  • If there is too little iodine in the diet, the body cannot make the thyroid hormone, and hypothyroidism will result.
  • If there is too much iodine, the production of thyroid hormone may be affected and, again, hypothyroidism can result.

In developed countries, iodine is added to regular table salt to ensure that individuals get enough iodine in their diets. Salt boxes are usually labeled "iodized salt."

In underdeveloped countries, there are nearly 200 million people with goiters (enlarged thyroids) due to insufficient iodine in their diet.

What Does Thyroid Hormone Do?

Cells respond to thyroid hormone with an increase in the rate at which they burn energy, called metabolic activity. Metabolic activity, or metabolism, is a term used to describe the processes in the body that produce energy and the chemical substances necessary for cells to grow, divide to form new cells, and perform other vital functions.

If you think of each cell in the body as a motor car, then thyroid hormone acts as if you were tapping on the accelerator pedal. Its message is "Go!"

Because thyroid hormone stimulates cells, it causes major body functions to "go" faster.

  • Heart rate increases.
  • Breathing rate increases.
  • Use of proteins, fats, and carbohydrates rises.
  • Skeletal muscles work more efficiently.
  • Muscle tone in the digestive system, such as those in the walls of the intestines that help to move food through the digestive system increases.
  • Mental alertness and thinking skills are sharpened.

How Are the Blood Levels of Thyroid Hormone Controlled?

Normally, levels of thyroid hormone are regulated so that the body runs like a car on cruise control, functioning at a steady rate. This steady state is known as homeostasis. Here's how the body's control system regulates the cells to function at a steady, appropriate metabolic rate:

  • Special "detector" cells in the brain monitor the level of thyroid hormone in the blood.
  • When the level of thyroid hormone drops, these cells send signals to a nearby organ in the brain known as the pituitary gland.
  • These signals stimulate the pituitary gland to release a substance called thyroid-stimulating hormone (TSH) into the bloodstream.
  • TSH signals cells in the thyroid gland to release more thyroid hormone into the bloodstream.
  • When the blood level of thyroid hormone has increased enough, the detector cells in the brain detect the increase in thyroid hormone level.
  • These detector cells send signals to the pituitary gland to stop release of TSH.

Nice To Know:

The medical specialty called endocrinology is devoted to the study and treatment of disorders of endocrine glands -- that is, glands that secrete hormones. Your primary care physician may refer you to an endocrinologist for a consultation or for ongoing care if you develop hypothyroidism or another endocrine condition.

Facts about hypothyroidism

  • Hypothyroidism can affect people of all ages, including children and infants.
  • More thann five million Americans have hypothyroidism.
  • Hypothyroidism affects both sexes, but women are up to eight times more likely to develop the condition.
  • As many as 10% of women may have some degree of thyroid hormone deficiency.
  • Millions of people have hypothyroidism but do not know it.
  • Hypothyroidism may affect between 4% and 9% of the general population, and between 9% and 16% of people over age 60.
  • Hypothyroidism is more common among people with diabetes
  • One out of every 4,000 infants is born with hypothyroidism.



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