The two most common causes of hypothyroidism are iodine deficiency and chronic thyroiditis. Iodine deficiency is rare in the United States and Europe but very common throughout the rest of the world. Chronic thyroiditis is an inherited condition that is diagnosed by checking the levels of thyroid autoantibodies in the blood.
In addition to these two causes, there are many other reasons that people become hypothyroid. The causes detailed in this article should be ruled out before your doctor starts treating your condition with thyroid hormone replacement.
Removal of the thyroid
If your thyroid has been removed because of cancer or an infection, or in the course of treatment for hyperthyroidism, you will usually become hypothyroid. Only if some tissue is left behind will the thyroid possibly continue to function.
Absence of brain hormones
Anything that destroys the hypothalamus (the part of the brain that secretes thyrotrophin-releasing hormone) or the pituitary gland at the base of the brain (which secretes thyroid-stimulating hormone, or TSH) will produce central hypothyroidism — hypothyroidism originating in the control center of the body, the brain. A trauma, infection, or infiltration (a replacement of brain tissue with other tissue, which can occur when a patient has cancer) could cause this type of destruction. The same result can occur if the pituitary is involved with a destructive lesion that prevents the production and release of TSH, such as radiation treatment to the area of the pituitary gland.
If hypothyroidism is caused by a problem with the hypothalamus or pituitary, some of the signs and symptoms associated with chronic (autoimmune) thyroiditis will not be found. In particular, hoarseness and a thickened tongue occur in autoimmune hypothyroidism but not in hypothyroidism associated with a lack of brain hormones. In addition, the thyroid is not usually enlarged in this instance, because TSH is not stimulating it. Also, the patient's hair and the skin are not coarse in this situation (but they are if the patient has autoimmune hypothyroidism).
Symptoms that result from a lack of other pituitary hormones also help to differentiate central hypothyroidism from failure of the thyroid gland. These include fine wrinkling of the skin of the face and a more pronounced loss of underarm, pubic, and facial hair.
Foods that cause hypothyroidism
Many common foods can cause hypothyroidism if you eat them in sufficient quantities, especially if you have an iodine deficiency. These foods are called goitrogens because they can trigger the enlargement of the thyroid (a goiter) as well as hypothyroidism. They block the conversion of T4 hormone to T3, the active form of thyroid hormone. Among the more common foods that cause this condition are
- Almond seeds
- Brussels sprouts
- Cabbage
- Cauliflower
- Corn
- Kale
- Turnips
If consuming these foods causes your condition, simply removing them from your diet will cure your hypothyroidism. It takes between three and six weeks for your thyroid to return to normal after you stop eating these foods.
Drugs that cause hypothyroidism
Many different medications cause hypothyroidism in the same way as the goitrogens listed in the previous section: They block the conversion of T4 to T3. The drugs you are most likely to run into include
- Adrenal steroids like prednisone and hydrocortisone, which treat inflammation
- Amiodarone, a heart drug
- Antithyroid drugs like propylthiouricil and methimazole
- Lithium, for psychiatric treatment
- Propranolol, a beta blocker
Coexisting autoimmune diseases
Occasionally, a patient with autoimmune thyroid disease has other autoimmune diseases, many of which involve other glands of the body. For example, diabetes mellitus type 1 sometimes occurs together with autoimmune thyroid disease. The cause is the autoimmune destruction of the insulin-producing cells of the pancreas. Another example is Addison's disease, the autoimmune destruction of the adrenal gland. Addison's disease is associated with severe fatigue and low blood pressure and is especially important to identify, because giving thyroid hormone without adrenal hormone to such a patient could be dangerous.
Autoimmune destruction of the ovaries in women or the testicles in men may also occur when a patient has autoimmune thyroiditis. The result for women is failure to menstruate, and for men it is infertility and impotency.
Another gland that may be affected by autoimmune disease is the parathyroid (which actually consists of four parathyroid glands) sitting behind the thyroid in the neck. Loss of parathyroid function results in low blood calcium and the possibility of severe muscle spasms and psychological changes.
Some autoimmune diseases that affect the joints of the body are found together with autoimmune thyroiditis. Rheumatoid arthritis is the most common example, but other diseases with names like Sjogren's syndrome and systemic lupus erythematosis are also diagnosed.
Be aware of a blood disease called pernicious anemia, an autoimmune disease that accompanies autoimmune thyroiditis on occasion. In this condition, cells of the stomach that produce acid are destroyed by autoimmunity. The patient is unable to absorb vitamin B12 and develops an anemia along with symptoms in the nervous system.
On occasion, when these diseases occur together, treatment of one of them treats the other at the same time. For example, treating the hypothyroidism with thyroid hormone may greatly improve the diabetes.
