Total T3 (Triiodothyronine)

What is total T3?

T3 (triiodothyronine) is the thyroid hormone with the strongest effect, regulating metabolism, heat production and heart rate at the cellular level. Part of it is secreted directly by the thyroid gland, while a larger part is formed in the tissues by conversion from T4. Maintaining body temperature, how fast the heart works and weight balance depend largely on the effect of this hormone. That is why symptoms such as palpitations, hot flashes and weight loss are prominent when there is too much T3.

Almost all T3 in the blood is carried bound to carrier proteins. The total T3 test measures this bound portion together with the small free portion, whereas the free T3 test shows only the free portion. Because the measurement method is well established and reliable, some doctors prefer total T3 when evaluating hyperthyroidism, but it should be kept in mind that the result is affected by the amount of carrier protein.

When is a total T3 test ordered?

Total T3 is mainly ordered when an overactive thyroid gland is suspected. If T4 values are normal while TSH is low, it helps determine whether the disease is presenting with T3 elevation alone. In some situations, the ratio between total T3 and total T4 can also give the doctor a clue in identifying the cause of hyperthyroidism. When the thyroid gland is underactive, on the other hand, total T3 can stay within normal limits for a long time, so it is rarely ordered for this purpose.

• Low TSH with normal T4
• Palpitations, weight loss, irritability and insomnia
• Assessing the severity of hyperthyroidism
• Monitoring during antithyroid treatment
• A free T3 result that does not match the clinical picture

How do I prepare for a total T3 test?

No fasting is required, and the blood sample can be given at any time of day. Because total T3 is affected by the carrier proteins in the blood, be sure to tell your doctor if you are pregnant or use estrogen-containing birth control pills or hormone therapy. Corticosteroid medications and some heart medications can also change the value. If you are going through a serious illness or have been eating very little for a long time, it is helpful to mention this as well, as it can also lower the value.

High-dose biotin (vitamin B7) supplements can make T3 appear higher than it actually is with some immunoassays; if you take such a supplement, ask your doctor when you should pause it. Choosing the same laboratory for measurements repeated for follow-up reduces variation caused by differences between methods. Blood can be drawn from the arm by a nurse at your home or workplace.

What does a total T3 result mean?

High total T3 usually suggests hyperthyroidism; in particular, high T3 while T4 is normal is known as T3 thyrotoxicosis. However, because pregnancy or estrogen use increases carrier proteins, total T3 can be high even when the thyroid gland is working normally. To make this distinction, your doctor may also order free T3 or free T4 measurements. A fall in total T3 during treatment may indicate a response to antithyroid treatment.

Low total T3, on the other hand, can be seen in serious illness, malnutrition, with some medications and when carrier proteins are low; it is not used on its own to diagnose hypothyroidism. Reference ranges vary by age, sex and laboratory; the range of the laboratory performing the test is shared on this page. Your total T3 result should be interpreted by your doctor together with TSH and your other thyroid tests, taking your complaints into account.