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Causes Of Hypothyroidism
By peace | March 18, 2007
Hypothyroidism is a decreased activity of the thyroid gland which may affect all body functions. The rate of metabolism slows causing mental and physical sluggishness. The most severe form of hypothyroidism is myxedema. Hypothyroidism can be caused by a problem with the thyroid itself (primary), or by the malfunction of the pituitary gland or hypothalamus (secondary).
Hypothyroidism is a very common condition. It is estimated that 3 to 5% of the population has some form of hypothyroidism. The condition is more common in women than in men, and its incidence increases with age. Worldwide, iodine deficiency remains the foremost cause of hypothyroidism. In the United States and other areas of adequate iodine intake, autoimmune thyroid disease is most common.
Hypothyroidism refers to any state in which thyroid hormone production is below normal. There are many disorders that result in hypothyroidism. These disorders 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.
Subclinical hypothyroidism refers to a state in which patients do not exhibit the symptoms of hypothyroidism. These patients also have a normal amount of circulating thyroid hormone. The only abnormality is an increased TSH on their 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. The majority of these patients can be expected to progress to obvious hypothyroidism, especially if the TSH is above a certain level.
The most common cause of hypothyroidism is an autoimmune reaction, where the body produces antibodies against the thyroid gland. One such autoimmune disorder is called Hashimoto’s thyroiditis, an inflammation of the thyroid gland. Other causes include treatment of hyperthyroidism such as radioactive iodine treatment or surgery.
Below is a list of some of the common causes of hypothyroidism:
1. Hashimoto’s Thyroiditis
The most frequent cause of acquired hypothyroidism is autoimmune thyroiditis (Hashimoto thyroiditis). 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. Blood samples drawn from patients with this disease reveal an increased number of antibodies to the enzyme, thyroid peroxidase (anti-TPO antibodies). Since the basis for autoimmune diseases may have a common origin, it is not unusual to find that a patient with Hashimoto’s thyroiditis has one or more other autoimmune diseases such as diabetes or pernicious anemia ( B12 deficiency). Hashimoto’s can be identified by detecting anti-TPO antibodies in the blood and by performing a thyroid scan.
2. Lymphocytic Thyroiditis After Hyperthyroidism
Thyroiditis refers to inflammation of the thyroid gland. When the inflammation is caused by a particular type of white blood cell known as a lymphocyte, the condition is referred to as lymphocytic thyroiditis. This condition is particularly common after pregnancy and can actually affect up to 8% of women after they deliver. In these cases, there is usually 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 6 months. The majority of affected women eventually return to a state of normal thyroid function, although there is a possibility of remaining hypothyroid.
3. Thyroid Destruction (from radioactive iodine or surgery)
Previous thyroid injury due to surgery or external irradiation (for head and neck neoplasms, breast cancer, or Hodgkin disease) may result in hypothyroidism and require monitoring. Patients who have been treated for a hyperthyroid condition (such as 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 is usually assumed that the thyroid will no longer function adequately. The result is hypothyroidism. Similarly, removal of the thyroid gland during surgery will be followed by hypothyroidism.
4. Pituitary 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, a decreased level of circulating T4 and T3 may result, even if the thyroid gland itself is normal. If this defect is caused by pituitary disease, the condition is called “secondary hypothyroidism”. If the defect is due to hypothalamic disease, it is called “tertiary hypothyroidism”.
A pituitary injury may result after brain surgery or if there has been a decrease of blood supply to the area. In these cases of pituitary injury, the TSH that is produced by the pituitary gland is deficient and blood levels of TSH are low. Because the thyroid gland is no longer stimulated by the pituitary TSH, hypothyroidism results. This form of hypothyroidism can, therefore, be distinguished from hypothyroidism that is caused by thyroid gland disease, in which the TSH level becomes elevated as the pituitary gland attempts to encourage thyroid hormone production by stimulating the thyroid gland with more TSH. Usually, hypothyroidism from pituitary gland injury occurs in conjunction with other hormone deficiencies, since the pituitary regulates other processes such as growth, reproduction, and adrenal function.
5. Medications
Medications that are used to treat an over-active thyroid (hyperthyroidism) may actually cause hypothyroidism. These drugs include methimazole (Tapezole) and propylthiouracil (PTU). The psychiatric medication, lithium, is also known to alter thyroid function and cause hypothyroidism. Interestingly, drugs containing a large amount of iodine such as amiodarone (Cardorone), SSKI, and Lugol’s solution can cause a decrease in thyroid function, thereby resulting in low blood levels of thyroid hormone.
6. Severe Iodine Deficiency
In areas of the world where there is an iodine deficiency in the diet, severe hypothyroidism can be seen in 5 to 15% of the population. Examples of these areas include Zaire, Ecuador, India, and Chile. Severe iodine deficiency is also seen 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 rarely seen in the United States.
7. Congenital hypothyroidism (present from birth).
Congenital hypothyroid child may present with slightly increased head size due to myxedema of the brain. There may be prolongation of physiological jaundice, lethargy, somnolence, large tongue and nasal obstruction. Affected infants cry little, sleep more and are very lethargic. There may be presence of umbilical hernia, hypothermia, constipation, edema of genitals and extremities, cardiomegaly, bradycardia and asymptomatic pericardial effusion.
8. Pregnancy
Some women develop hypothyroidism during or after pregnancy (postpartum hypothyroidism), often because they produce antibodies to their own thyroid gland. Left untreated, hypothyroidism increases the risk of miscarriage, premature delivery and preeclampsia — a condition that causes a significant rise in a woman’s blood pressure during the last three months of pregnancy. It can also seriously affect the developing fetus.le that they go unnoticed.
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