There are many reasons for low free T4, such as hypothyroidism, Hashimoto’s thyroiditis, application of antithyroid medication, etc. It is recommended to consult a doctor promptly. 1. Primary hypothyroidism: low free T4, low free T3 and high TSH can be seen in primary hypothyroidism, which is a decrease in thyroid hormone secretion caused by the thyroid gland itself, and should be treated with levothyroxine sodium tablets. 2. Central hypothyroidism: low free T4, low free T3, low TSH, may be central hypothyroidism, often due to pituitary lesions, resulting in reduced TSH secretion, which in turn inhibits thyroid hyperplasia, resulting in reduced secretion of thyroid hormones, and should be given levothyroxine sodium tablets as a replacement therapy, or surgical treatment if necessary. If the free T4 is low, you should go to the hospital to find out the cause, and treat according to the cause of the disease under the guidance of the doctor.