Free triiodothyronine (FT3) is an important component of thyroid hormone, and its low level is often seen in primary hypothyroidism, central hypothyroidism and low T3 syndrome. 1. Primary hypothyroidism: Hypothyroidism is caused by the lesion of the thyroid gland itself. In the late stage, low FT3 is accompanied by a decrease in free thyroxine (FT4) and an increase in thyroid stimulating hormone (TSH), and low metabolism can be seen, such as dullness of sensation and slow movement, accompanied by mucous edema, and so on. 2. Central hypothyroidism: hypothyroidism caused by hypothalamic-pituitary pathology, such as tumors and inflammation of the hypothalamus and pituitary gland, causing insufficient secretion of TSH, which in turn leads to a decrease in FT3 and FT4. 3. Low T3 syndrome: it refers to the impact of various non-thyroidal diseases, such as hypocorticism and hepatic insufficiency, on thyroid function, which slows down the conversion of T4 to T3, and manifests itself as a situation where only T3 is lowered. In conclusion, if you have low free triiodothyronine, you should go to the cause in time, improve the relevant examination, and carry out the treatment under the doctor’s guidance.