Thyroid Screening

WHAT IS THE THYROID GLAND?

Just as the thyroid gland communicates with other organs through the hormone it produces, the pituitary gland in the brain communicates with the thyroid through a hormone it makes—thyroid-stimulating hormone, or TSH. When the pituitary senses that thyroid hormone levels are too low, it releases more TSH to coax the thyroid into action. When the thyroid is nudged by TSH, it produces thyroid hormone—a large proportion of which is thyroxine (T4) and a smaller proportion triiodothyronine (T3). The T4 is eventually converted into T3, the “active” form that is taken up by receptors in body cells.

MONITORING THE THYROID GLAND

A blood test for levels of TSH is the most sensitive test for determining whether you have hypothyroidism. Most laboratories use 0.45 – 5.00 mIU/L as a normal reference range for TSH. People with TSH between 5.00 and 9.99 mIU/L often have no symptoms (known as subclinical hypothyroidism), but some do. Another test called T4 will be done if your TSH is in this range. A low level of T4 usually means you will benefit from thyroid hormone replacement.

Many people with hypothyroidism or subclinical hypothyroidism aren’t aware anything is wrong because they haven’t been tested. The U.S. Preventive Services Task Force (USPSTF) doesn’t recommend thyroid screening because it hasn’t found sufficient evidence that testing thyroid hormone levels in large groups of people without symptoms is cost-effective. Dr. Garber agrees, and suggests a different approach—testing asymptomatic people who are most likely to develop thyroid disease and benefit from treatment.

Treating subclinical hypothyroidism with synthetic thyroid hormone may reduce the risk of developing more serious problems like cardiovascular disease. They note that low thyroid hormone can cause a high cholesterol level and treatment with thyroid hormone may make statin therapy unnecessary.

HOW DOES THE THYROID GLAND FUNCTION?

The major thyroid hormone secreted by the thyroid gland is thyroxine, also called T4 because it contains four iodine atoms. To exert its effects, T4 is converted to triiodothyronine (T3) by the removal of an iodine atom. This occurs mainly in the liver and in certain tissues where T3 acts, such as in the brain. The amount of T4 produced by the thyroid gland is controlled by another hormone, which is made in the pituitary gland located at the base of the brain, called thyroid stimulating hormone (abbreviated TSH). The amount of TSH that the pituitary sends into the bloodstream depends on the amount of T4 that the pituitary sees. If the pituitary sees very little T4, then it produces more TSH to tell the thyroid gland to produce more T4. Once the T4 in the bloodstream goes above a certain level, the pituitary’s production of TSH is shut off.

SYMPTOMS

Hypothyroidism signs and symptoms may include:

• Fatigue
• Increased sensitivity to cold
• Constipation
• Dry skin
• Weight gain
• Puffy face
• Hoarseness
• Muscle weakness
• Elevated blood cholesterol level
• Muscle aches, tenderness and stiffness
• Pain, stiffness or swelling in your joints
• Heavier than normal or irregular menstrual periods
• Thinning hair
• Slowed heart rate
• Depression
• Impaired memory
• Enlarged thyroid gland (goiter)

Hyperthyroidism can mimic other health problems, which can make it difficult for your doctor to diagnose. It can also cause a wide variety of signs and symptoms, including:

• Unintentional weight loss, even when your appetite and food intake stay the same or increase
• Rapid heartbeat (tachycardia) — commonly more than 100 beats a minute
• Irregular heartbeat (arrhythmia)
• Pounding of your heart (palpitations)
• Increased appetite
• Nervousness, anxiety and irritability
• Tremor — usually a fine trembling in your hands and fingers
• Sweating
• Changes in menstrual patterns
• Increased sensitivity to heat
• Changes in bowel patterns, especially more frequent bowel movements
• An enlarged thyroid gland (goiter), which may appear as a swelling at the base of your neck
• Fatigue, muscle weakness
• Difficulty sleeping
• Skin thinning
• Fine, brittle hair

TESTS

Blood tests to measure these hormones are readily available and widely used, but not all are useful in all situations. Tests to evaluate thyroid function include the following:

• TSH TESTS
  The best way to initially test thyroid function is to measure the TSH level in a blood sample. Changes in TSH can serve as an “early warning system” – often occurring before the actual level of thyroid hormones in the body becomes too high or too low. A high TSH level indicates that the thyroid gland is not making enough thyroid hormone (primary hypothyroidism). The opposite situation, in which the TSH level is low, usually indicates that the thyroid is producing too much thyroid hormone (hyperthyroidism). Occasionally, a low TSH may result from an abnormality in the pituitary gland, which prevents it from making enough TSH to stimulate the thyroid (secondary hypothyroidism). In most healthy individuals, a normal TSH value means that the thyroid is functioning properly.
• T4 TESTS
  T4 is the main form of thyroid hormone circulating in the blood. A Total T4 measures the bound and free hormone and can change when binding proteins differ (see above). A Free T4 measures what is not bound and able to enter and affect the body tissues. Tests measuring free T4 – either a free T4 (FT4) or free T4 index (FTI) – more accurately reflect how the thyroid gland is functioning when checked with a TSH.

The finding of an elevated TSH and low FT4 or FTI indicates primary hypothyroidism due to disease in the thyroid gland. A low TSH and low FT4 or FTI indicates hypothyroidism due to a problem involving the pituitary gland. A low TSH with an elevated FT4 or FTI is found in individuals who have hyperthyroidism.

• T3 TESTS
  T3 tests are often useful to diagnose hyperthyroidism or to determine the severity of the hyperthyroidism. Patients who are hyperthyroid will have an elevated T3 level. In some individuals with a low TSH, only the T3 is elevated and the FT4 or FTI is normal. T3 testing rarely is helpful in the hypothyroid patient, since it is the last test to become abnormal. Patients can be severely hypothyroid with a high TSH and low FT4 or FTI, but have a normal T3.
• FREE T3
  Measurement of free T3 is possible, but is often not reliable and therefore not typically helpful.
• REVERSE T3
  Reverse T3 is a biologically inactive protein that is structurally very similar to T3, but the iodine atoms are placed in different locations, which makes it inactive. Some reverse T3 is produced normally in the body, but is then rapidly degraded. In healthy, non-hospitalized people, measurement of reverse T3 does not help determine whether hypothyroidism exists or not, and is not clinically useful.

TREATMENT

Several options for treatment of both hypo and hyperthyroidism are available. Once your test results are available, your healthcare provider will go over your results as well as specific treatment options for your specific case with you