Low T3 syndrome is a condition in which the TSH is normal and the T4 is normal, but only the T3 is reduced. Low T3 syndrome occurs mainly because of the following reasons: 1, under normal circumstances in the peripheral blood plays a role in T3, and the thyroid gland secretes hormones mainly T4, so the thyroid gland secretes T4 must be converted to T3 in the peripheral blood to play a role. 2, when the body occurs serious diseases, such as systemic lupus erythematosus, rheumatoid arthritis, serious infections, heart, liver and kidney insufficiency and other diseases and diseases state In order to reduce the metabolic rate, the body will reduce the hormone synthesized by the thyroid gland into T3, and more into anti-T3, which does not work in the peripheral blood. Therefore, low T3 syndrome is also known as a sick syndrome with normal thyroid function. Low T3 syndrome does not require treatment, and will improve on its own after the primary disease is cured.