Why Is Thyroid Stimulating Hormone Measured?
Thyroid stimulating hormone is one of the hormones involved in the hypothalamic-pituitary-thyroid axis, which ultimately regulates our body's metabolism. The hypothalamus secretes thyroid releasing hormone (TRH), which causes the pituitary gland to secrete thyroid stimulating hormone (TSH). Thyroid stimulating hormone causes the thyroid hormones, T3 and T4 to be released, and these control metabolic functions within the cells. When too much T3 or T4 is circulating, it shuts off production of TRH, so the process is controlled by a negative feedback loop.
Why Thyroid Stimulating Hormone?
Thyroid function can be monitored by measuring T3, T4 and/or TSH. TSH is the best indicator of thyroid function. It helps determine where a thyroid problem is located—the thyroid gland or the pituitary. It gives an earlier indication of thyroid problems than T3 or T4
An elevated TSH indicates that the thyroid gland is not secreting enough thyroid hormones (hypothyroidism). A low thyroid stimulating hormone level indicates the thyroid is producing too much hormone (hyperthyroidism).
What Is Normal?
The normal range for thyroid stimulating hormone used to be 0.5-5.0. Recently, the "normal" has changed. TSH levels above 3.0 are now considered to be an indication of hypothyroidism.
This came about for two reasons. First, doctors noted that some patients have clinical signs and symptoms of thyroid disease even with normal thyroid stimulating hormone levels. This made them wonder if the "normal" was wrong. Then they noticed that people with TSH levels above 3.0 almost always had rising TSH levels, and they usually rose to above 5.0.
By changing the "normal" upper limit for thyroid stimulating hormone levels, endocrinologists estimate than an additional 20% of their patients will be diagnosed as being hypothyroid.
They changed the "normal" because they determined that people needed to be treated for hypothyroidism when their TSH level is above 3.0. This keeps it from getting worse, that is, continuing to rise until it is 5.0. Earlier treatment for hypothyroidism also relieves subclinical symptoms such as fatigue, mental fogginess and dry hair and skin.
Hypothyroidism means not enough thyroid hormone is being produced by the body. The thyroid stimulating level is above 3.0. The most common cause of hypothyroidism is Hashimoto's thyroiditis, an autoimmune disorder where the body attacks and destroys the thyroid gland.
The signs of hypothyroidism are fatigue, feeling cold constantly, weight gain, puffiness, depression, constipation. Hypothyroidism—not enough thyroid hormone—causes all body processes to slow down. Hypothyroidism is treated by taking synthetic thyroid hormone.
Hyperthyroidism (too much thyroid hormone) is diagnosed when the TSH level is below 0.3. It is usually caused by Grave's disease, another autoimmune disorder. Symptoms of hyperthyroidism are related to "sped up" metabolic processes. People experience diarrhea, rapid heart rate, anxiety, irritability and weight loss.
Hyperthyroidism is treated by taking thyroid suppressing medications, by destroying part of the gland with radioactive iodine or by surgically removing part of the thyroid gland.
Thyroid function tends to decline with age, but our need for thyroid hormone does not. Most doctors begin to check the thyroid stimulating hormone level every year or two after you reach about age 40.