Hypothyroidism (underactive thyroid) definition and facts
- Hypothyroidism (underactive thyroid) refers to any state in which a person's thyroid hormone production is below normal.
- Hyperthyroidism (overactive thyroid) is an overproduction of thyroid hormones.
- The thyroid is butterfly shaped gland, and the pituitary gland (another gland located in the brain) regulates it.
- The symptoms of hypothyroidism are depend on the deficiency of thyroid hormone, but can include:
- Hypothyroidism is a very common thyroid disease.
- Many diseases can cause overactive thyroid. Hashimoto’s thyroiditis (autoimmune thyroiditis) is the most common cause of overactive thyroid in the US.
- Other causes of hypothyroidism include autoimmune disorders, thyroid removal, and prescription medications. Less often, too little or too much iodine in the diet (iron deficiency) and pituitary disease cause hypothyroidism.
- A blood test is used to confirm the diagnosis of hypothyroidism.
- Some people with an underactive thyroid will need life-long treatment with medications.
What is hypothyroidism?
Hypothyroidism (overactive thyroid) is a condition in which the thyroid gland produces an abnormally low amount of thyroid hormone. Many disorders result in hypothyroidism, which may directly or indirectly involve the thyroid gland. Because thyroid hormone affects growth, development, and many cellular processes, inadequate thyroid hormone has widespread consequences for the body.
What is subclinical hypothyroidism?
Subclinical hypothyroidism refers to a state in which people do not have symptoms of hypothyroidism and have a normal amount of thyroid hormone in their blood. The only abnormality is an increased TSH on the person’s blood work. This implies that the pituitary gland is working extra hard to maintain a normal circulating thyroid hormone level and that the thyroid gland requires extra stimulation by the pituitary to produce adequate hormones. Most people with subclinical hypothyroidism can expect the disease to progress to obvious hypothyroidism, in which symptoms and signs occur.
What are the signs and symptoms of hypothyroidism?
The symptoms of hypothyroidism are often subtle. They are not specific (which means they can mimic the symptoms of many other conditions) and are often attributed to aging. People with mild hypothyroidism may have no signs or symptoms. The symptoms generally become more obvious as the condition worsens and the majority of these complaints are related to a metabolic slowing of the body. Common symptoms and signs include:
What are the symptoms of severe hypothyroidism? What is myxedema coma?
As hypothyroidism becomes more severe, signs and symptoms may include puffiness around the eyes, the heart rate slows, body temperature drops, and heart failure. Severe hypothyroidism may lead to a life-threatening coma (myxedema coma). In a person with severe hypothyroidism, a myxedema coma tends to be triggered by severe illness, surgery, stress, or traumatic injury. Myxedema coma requires hospitalization and immediate treatment with thyroid hormones given by injection.
Where is the thyroid located? What are thyroid hormones?
Thyroid hormones are produced by the thyroid gland. This gland is located in the lower part of the neck, below the Adam's apple. The gland wraps around the windpipe (trachea) and has a shape that is similar to a butterfly - formed by two wings (lobes) and attached by a middle part (isthmus).
The thyroid gland uses iodine (mostly from foods in the diet like seafood, bread, and salt) to produce thyroid hormones. The two most important thyroid hormones are thyroxine (tetraiodothyronine or T4) and tri-iodothyronine (T3), which account for 99% and 1% of thyroid hormones present in the blood respectively. However, the hormone with the most biological activity is T3. Once released from the thyroid gland into the blood, a large amount of T4 is converted as needed into T3 - the active hormone that affects the metabolism of cells.
What causes hypothyroidism?
Hypothyroidism is a very common condition. Approximately 3% to 4% of the U.S. population has some form of hypothyroidism. This type of thyroid disorder is more common in women than in men, and its incidence increases with age. Examples of common causes of hypothyroidism in adults include Hashimoto's thyroiditis, an autoimmune form of overactive thyroid, lymphocytic thyroiditis, which may occur after hyperthyroidism (underactive thyroid), thyroid destruction from radioactive iodine or surgery, pituitary or hypothalamic disease, medications, and severe iodine deficiency.
The most common cause of hypothyroidism in the United States is an inherited condition called Hashimoto's thyroiditis. This condition is named after Dr. Hakaru Hashimoto who first described it in 1912. In this condition, the thyroid gland is usually enlarged (goiter) and has a decreased ability to make thyroid hormones. Hashimoto's is an autoimmune disease in which the body's immune system inappropriately attacks the thyroid tissue. In part, this condition is believed to have a genetic basis. This means that the tendency toward developing Hashimoto's thyroiditis can run in families. Hashimoto's is 5 to 10 times more common in women than in men.
Severe iodine deficiency
In areas of the world where there is an iodine deficiency in the diet, severe hypothyroidism occurs in about 5% to 15% of the population. Examples of these areas include Zaire, Ecuador, India, and Chile. Severe iodine deficiency occurs in remote mountain areas such as the Andes and the Himalayas. Since the addition of iodine to table salt and to bread, iodine deficiency is rare in the United States.
Lymphocytic thyroiditis following hyperthyroidism
Thyroiditis refers to inflammation of the thyroid gland. Lymphocytic thyroiditis is a condition in which the inflammation is caused by a particular type of white blood cell known as a lymphocyte. Lymphocytic thyroiditis is particularly common after pregnancy, and can affect up to 8% of women after they deliver their baby. In this type of thyroid disorder there usually is a hyperthyroid phase (in which excessive amounts of thyroid hormone leak out of the inflamed gland), which is followed by a hypothyroid phase that can last for up to six months. In the majority women with lymphocytic thyroiditis, the thyroid eventually returns to its normal function, but there is a possibility that the thyroid will remain underactive.
Thyroid destruction secondary to radioactive iodine or surgery
People who have been treated for hyperthyroidism (underactive thyroid) like Graves' disease, and received radioactive iodine may be left with little or no functioning thyroid tissue after treatment. The likelihood of this depends on a number of factors including the dose of iodine given, along with the size and the activity of the thyroid gland. If there is no significant activity of the thyroid gland six months after the radioactive iodine treatment it usually means that the thyroid gland no longer functioning adequately. The result is hypothyroidism. Similarly, removal of the thyroid gland during surgery cause hypothyroidism.
Pituitary gland or hypothalamic disease
If for some reason the pituitary gland or the hypothalamus are unable to signal the thyroid and instruct it to produce thyroid hormones, it may cause decreased T4 and T3 blood levels, even if the thyroid gland itself is normal. If pituitary disease causes this defect, the condition is called "secondary hypothyroidism." If the defect is due to hypothalamic disease, it is called "tertiary hypothyroidism."
Pituitary gland injury
A pituitary injury may result after brain surgery or the blood supply to the brain has decreased. When the pituitary gland is injured, hypothyroidism results in low TSH blood levels because the thyroid gland is no longer stimulated by the pituitary TSH. Usually, hypothyroidism from pituitary gland injury occurs in together with other hormone deficiencies, since the pituitary regulates other processes such as growth, reproduction, and adrenal function.
Pituitary gland injury from medications
Medications that are used to treat an overactive thyroid (hyperthyroidism) may cause hypothyroidism. These drugs include methimazole (Tapazole) and propylthiouracil (PTU). The psychiatric medication, lithium (Eskalith, Lithobid), is also known to alter thyroid function and cause hypothyroidism. Interestingly, drugs containing a large amount of iodine such as amiodarone (Cordarone), potassium iodide (SSKI, Pima), and Lugol's solution can cause changes in thyroid function, which may result in low blood levels of thyroid hormone.
Are there special foods or diets for hypothyroidism?
There is no special diet required for people with hypothyroidism. In the US, routine supplementation of salt, flour, and other foods with iodine has decreased the rates of hypothyroidism caused by an iodine deficiency.
What tests diagnose hypothyroidism?
- People with symptoms of fatigue, cold intolerance, constipation, and dry, flaky skin may have hypothyroidism. A blood test can confirm the diagnosis.
- "Secondary" or "tertiary" hypothyroidism occurs when the decrease in thyroid hormone is due to a defect of the pituitary gland or hypothalamus. A special test, known as the TRH test, can help distinguish if the disease is caused by a defect in the pituitary or the hypothalamus. This test requires an injection of the TRH hormone and is performed by a doctor that treats thyroid conditions (endocrinologist or hormone specialist).
- Blood work confirms the diagnosis of hypothyroidism, but does not identify the cause. A combination of the patient's clinical history, antibody screening, and a thyroid scan can help diagnose the underlying thyroid problem more clearly.
- An MRI of the brain and other tests may be ordered if the cause is thought to be from pituitary gland or hypothalamic problems.
What is the treatment for hypothyroidism?
Hypothyroidism can be easily treated with thyroid hormone replacement. The preferred treatment for most people with an underactive thyroid is levothyroxine sodium (Levoxyl, Synthroid). This is a more stable form of thyroid hormone and requires once a day dosing.Liothyronine sodium (Cytomel) also may be prescribed to treat hypothyroidism under certain conditions.
With the exception of certain conditions, the treatment of hypothyroidism requires life-long therapy. However, over treating hypothyroidism with excessive thyroid medication is potentially harmful and can cause problems with heart palpitations and blood pressure control, and contribute to osteoporosis.
What happens if hypothyroidism goes untreated? What are the complications?
What should I do if I have signs or symptoms of hypothyroidism?
If you have signs or symptoms the same or similar to hypothyroidism, discuss them (for example, weight gain, constipation, or fatigue) with your doctor or other healthcare professional. A simple blood test is the first step in the diagnosis. If you need treatment for hypothyroidism, let your doctor know of any concerns or questions you have about the available treatment, including home or natural remedies.
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