Low free thyroxine level is often closely related to abnormal thyroid function and pituitary gland function, and the main causes include primary hypothyroidism, thyroid gland damage, pituitary gland damage, and so on. 1. Primary hypothyroidism: Low free thyroxine level may be caused by the reduced function of thyroid follicular cells and decreased hormone secretion. 2. Thyroid gland damage: mainly including subacute thyroiditis and Hashimoto’s disease, etc., which leads to the damage of thyroid follicular cells, and further leads to the reduction of thyroid hormone synthesis, causing corresponding changes. 3. Pituitary damage: when the pituitary gland is damaged, the level of thyroid stimulating hormone secreted by the pituitary gland decreases, which indirectly leads to a decrease in the synthesis of thyroxine by the follicular cells of the thyroid gland, thus causing the corresponding changes. Once the level of free thyroxine is low, it is necessary to consult a doctor in time to adjust the condition and supplement thyroxine tablets if necessary to prevent affecting the metabolism of the body and causing physical damage.