Low FT4 free thyroxine is mainly seen in hypothyroidism, application of certain drugs and other conditions. 1. Hypothyroidism: It is a generalized hypometabolic syndrome caused by low serum thyroid hormone levels or thyroid hormone resistance that cannot play its corresponding role. FT4 free thyroxine is a kind of thyroid hormone, so it will appear in the situation of low levels. For hypothyroidism, it is necessary to follow the doctor’s instructions to supplement thyroxine treatment, commonly used drugs are levothyroxine sodium tablets, thyroxine tablets, etc., should be regularly recheck the thyroid function and adjust the dose of drugs. 2. Application of certain drugs: some patients applying certain drugs, such as methimazole, methylthiouracil and other drugs for hyperthyroidism, will lead to low FT4 free thyroxine, which will be relieved after adjusting the dose of the drug or stopping the drug. When the FT4 free thyroxine is low, it is necessary to combine the patient’s medical history, medication history, and other tests to clarify the cause of the disease, and timely treatment.