Interpreting TSH results is essential for diagnosing and treating thyroid dysfunction. For some patients, TSH does not tell the whole story. Thyroid hormones, T4 and T3 are directly produced by the thyroid gland. Both can be measured in blood in several different ways, all of which are helpful in managing thyroid abnormalities like hypothyroidism (underactive thyroid) and hyperthyroidism (overactive thyroid).
T4
T4 is also known as thyroxine or tetraiodothyronine. The latter is a long, fancy way of saying T4 contains four iodine molecules. More T4 is made in the thyroid than T3. A synthetic form of T4, levothyroxine, is the mainstay of treatment in hypothyroidism. T4 made by the patient's body cannot be distinguished from T4 provided by medication. T4 itself has very little activity. Its main function is to be converted to T3, which is the more active thyroid hormone used by the body's cells. There are several ways to measure T4 in the blood.
Total T4
Total T4 assesses the complete amount of thyroxine in the blood. This includes T4 that is carried on proteins and free T4 that floats around in the blood, available to be used for conversion to T3. Because T4 is bound on proteins, changes in these protein levels can affect total T4 levels. In patients taking estrogen, such as with hormone replacement therapy or birth control pills, total T4 may be falsely elevated due to changes in the carrier proteins. This also happens during pregnancy. Hyperthyroidism causes true elevations in total T4, while patients with untreated hypothyroidism may show low levels of total T4.
The normal range for total T4 is approximately 4.8 to 10.4 mcg/dl in adults. A higher level is considered normal in pregnancy due to the changes in protein-binding.
Free T4
The free T4 test measures the T4 that's not bound on proteins in the blood, eliminating the protein interference problems. For that reason, it can be a more useful test than total T4. Free T4 is often measured along with TSH to help manage hypothyroidism.
A low free T4 may indicate that the patient is hypothyroid or needs more T4 hormone. However, if the patient is also taking a medication that includes T3, the free T4 level could be artificially low. In this case, low free T4 does not indicate the need for more thyroid hormone. A hypothyroid patient with a high free T4 level may be receiving too much thyroid hormone and often requires a decrease in dose.
Patients with hyperthyroidism usually will have a high free T4 level. This generally is the first thing to improve when people start treatment with antithyroid medication. Doses of antithyroid medication may be adjusted when the free T4 and free T3 reach the normal range, even if the TSH is still low.
The normal range for free T4 is about 0.8 to 1.7 mcg/dl.
T3
T3 is also known as triiodothyronine, meaning it is composed of three iodine molecules. Only about 20% of T3 is made in the thyroid. The rest comes from conversion of T4 throughout the body. T3 is the active form of thyroid hormone that is used by the cells. It does not last as long in the body as T4. T3 is also available as a synthetic medication, liothyronine (Cytomel), which can be used for treatment of hypothyroidism.
Total T3 and Free T3
T3 levels are not particularly helpful in diagnosing or managing hypothyroidism. They really come into play with hyperthyroidism. Both levels are high in hyperthyroidism. Free T3 is used more often for the same reasons that free T4 is used, elimination of protein interference. The test is very useful in adjusting antithyroid medications during treatment of hyperthyroidism. When both free T4 and free T3 have reached the normal range, the dose of antithyroid medication may be lowered.
The normal range for total T3 is approximately 97 to 219 ng/dl and for free T3 is 210 to 440 pg/dl.
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