Triiodothyronine (T3) belongs to a kind of thyroid hormone. A low level of this indicator means that the level of this hormone detected in the blood is lower than normal, which can be seen in hypothyroidism, low T3 syndrome and other diseases. 1. Hypothyroidism: Hypothyroidism is a lack of thyroid hormone or insufficient biological effect caused by different reasons, which can be manifested by symptoms such as loss of appetite, fear of cold, bradycardia, dry skin and mental depression. Thyroid function tests may reveal a decrease in triiodothyronine, and it is recommended to combine with other indicators of thyroid function to make a definite diagnosis. 2. Low T3 syndrome: severe systemic diseases (such as malignant tumors, severe malnutrition, severe infections), trauma, surgery, drugs and other factors may lead to changes in the level of thyroid hormones, triiodothyronine is low, at this time, the manifestation of hypothyroidism is not obvious, and it is mainly manifested as the primary symptoms. When low triiodothyronine is detected, you should consult a doctor in time to find out the cause and further diagnosis and treatment by a professional doctor.