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How Does a Doctor Determine You Need Thyroid Hormones?
After you have discussed your symptoms with your doctor and hypothyroidism
is suspected, a blood test for TSH (thyroid stimulating hormone), and possibly others, may be ordered. When a physician receives a laboratory
report with the result, the report slip will state normal ranges for TSH and possibly for other
specific hormones with the result. The TSH measurement is of fundamental importance since it reflects
the sufficiency of the body’s supply of thyroid hormone. A normal-range TSH indicates
that the hypothalamus senses a normal amount
of thyroid hormone and is stimulating the thyroid to continue making and releasing
thyroid hormone at the same rate. An elevated TSH level signals the insufficiency of thyroid hormone, while a reduced TSH level indicates
increased amounts of thyroid hormone. There are many blood tests used to diagnose and monitor patients with thyroid conditions. No single lab test is considered 100% accurate in determining thyroid disease, but a combination of two or more tests may help to determine if there is a problem with the thyroid gland or the pituitary gland which stimulates it. The Thyroid Stimulating Hormone (TSH) and Free Thyroxine Index (FT4I) are the two blood tests used to diagnose patients with hypothyroidism and to monitor their progress while using a thyroid medication.1 However, in many situations, it is necessary to perform other tests as well. Several hormones, their relationship to one another, and the respective tests used to measure the hormone levels are described below: Thyroid Stimulating Hormone (TSH)1: TSH, also referred to as thyrotropin, is released by the pituitary gland in response to TRH stimulation. Just as the name implies, TSH stimulates the thyroid gland to produce more thyroid hormone for the body. In summary, a low level of circulating thyroid hormone results in a release of TRH, which stimulates TSH production, and finally an increased production of thyroid hormones by the thyroid gland. TSH measurement may help the physician to determine if the problem is primary (thyroid gland) or secondary (pituitary gland) hypothyroidism. TSH measurement may also be used, in conjunction with other tests, to determine whether the dose of thyroid supplementation medication needs to be adjusted. For example, if the TSH level is high, an increase in thyroid supplement dose may be needed. Total T4 Immunoassay (Total T4, T4 RIA, thyroxine, or T4)1: This test is used to determine the total amount of T4 in the blood, representing both the bound (to proteins) and unbound portions of T4. The majority of T4 in the blood is bound to protein and therefore has no activity. The amount of free versus bound T4 can be measured with a separate test. Free Thyroxine Index (FT4I)1: This test is used to determine the amount of free (unbound) T4 in the blood. It may be important to look at the amount of free T4 since this is the portion of thyroid hormone available to become active in the body. Total T3 Immunoassay (Total T3, T3RIA, L-triiodothyronine, or T3)1: This test is used to determine the total amount (both bound and unbound portions) of T3—the more potent and active form of the two thyroid hormones—in the blood. Like T4, if the hormone is bound to protein, it is considered inactive. This test is most often used in the diagnosis of different types of hyperthyroidism, such as Graves’ disease. In hypothyroidism, the T3 level may often remain within the normal range. Free T3: It is believed that the minute amount of free hormone (the amount not bound by proteins in the circulation) may be the amount that is responsible for the biological activity of thyroid hormones at the cellular level (0.02% for Free T4 versus 0.2% for Free T3).2 Measuring the Free T3 value involves either two-test index methods, physical separation methods that isolate free from protein-bound hormone, or immunoassay methods. Your physician may have a preference for which Free T3 test to use. Normal values for thyroid blood tests may vary with different laboratories. It is always important to compare your results with the normal values provided by your laboratory. It is also important to pay particular attention to the units of measure (ie, µg/dL versus mmol/L) to be sure you are comparing similar entities. The following are average normal values1: |
Wide Range of Variation When a patient has symptoms suggestive of hypothyroidism but is shown to have lower normal values of thyroid hormone, a physician may start a trial of thyroid hormone therapy, hoping to increase these lower normal thyroid hormone values to upper normal values. If there is a sustained favorable response without any evidence of clinical hyperthyroidism, long-term replacement therapy may be justified.
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