The thyroid gland is located in the front of the neck and is responsible for controlling many body functions including the functioning of other endocrine glands and lipid levels. Measurement of thyroid function requires a blood test for levels of thyroid stimulating hormone (TSH) and thyroxin (T4), otherwise known as thyroid hormone. When the brain detects low levels of T4 it secretes more TSH in order to have the thyroid gland make more T4. Screening for thyroid function should also include levels of anti-thyroid antibodies (i.e. thyroid peroxidase antibody). Hashimoto’s thyroiditis, an autoimmune inflammatory disease of the thyroid, accounts for 60 to 80 percent of hypothyroidism (underactive thyroid) in people who have positive levels of anti-thyroid antibodies. As we will see, recommendations for thyroid screening and when to begin treatment of hypothyroidism vary greatly.
The most common scenario encountered in clinical practice is subclinical hypothyroidism. This is when a person has mildly elevated levels of TSH, normal T4, and no symptoms. Estimates put the incidence of subclinical hypothyroidism at between four to 20 percent of the U.S. population. When testing for TSH levels, it is important to take the time of day into account. TSH secretion is pulsatile and circadian. TSH levels can be 50 percent higher at night and in the early morning than they are during the day. “Normal” TSH levels are between four and five, depending on the lab’s reference values. Someone with a TSH up to 10 but a normal T4 with no symptoms has subclinical hypothyroidism. About 60 percent of these people may revert to a euthyroid (normal) state within five years without treatment, while about five percent will go on to develop overt (symptomatic) hypothyroidism. Most of those who progress to overt disease test positive for anti-thyroid antibodies.
The decision as to whether to treat someone who has subclinical hypothyroidism has many factors. These include a history of thyroid or neck surgery, a history of neck irradiation, a history of infiltrative diseases (amyloidosis, sarcoidosis, hemochromatosis), a family history of thyroid disease, and use of certain drugs like lithium and intravenous contrast dye. While the recommendations on treatment, like screening recommendations, vary widely, many experts agree that if the TSH is between five and 10 and there are anti-thyroid antibodies and/or risk factors present, treatment with levothyroxine should begin.
As far as screening decisions go, risk factors for hypothyroidism, such as a family history of it, neck irradiation, thyroid surgery, elevated lipid levels, atrial fibrillation, autoimmune disorders, unexplained weight loss, and the use of certain medications must be considered. The American Thyroid Association recommends screening adults starting at age 35 with repeat screening every five years. The American Academy of Family Physicians recommends starting screening after age 60, and the American College of Physicians advises after age 50. As I said, screening recommendations vary widely. Since over 70 percent of those with subclinical hypothyroidism have no symptoms, it is reasonable to base screening on risk. Someone with positive risk factors should probably start screening between 35 and 40. And, as always, screening should include looking for anti-thyroid antibodies.
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