In general, low thyroid hormones represent low thyroid function, also known as hypothyroidism. Common clinical causes are autoimmune diseases, such as Hashimoto’s thyroiditis, which can lead to low thyroid function. There is also the result of radioactive iodine treatment, which causes hypothyroidism due to radiation killing functional cells of the thyroid gland. Another common one is surgical treatment to cut off part of the thyroid gland, which causes hypothyroidism. In clinical practice, patients with hypothyroidism are usually given thyroid hormone supplements and monitored regularly. There is also a special condition called low T3 syndrome, which often occurs along with serious illness. This condition usually does not require thyroid hormone replacement, so when low thyroid hormone levels are present, an endocrinology visit is required.