Free triiodothyronine is the physiologically active part of thyroxine and is one of the indicators of thyroid function. When abnormal thyroid function is suspected, both serum free thyroxine and thyroid stimulating hormone should be tested. When free triiodothyronine and free thyroid hormone are increased, while thyroid stimulating hormone is decreased, primary thyrotoxicosis is diagnosed, which is seen in Graves’ disease, hyperthyroidism, as well as transient thyrotoxicosis after destruction of thyroid cells caused by various thyroiditis, high-functioning adenoma of the thyroid gland and Thyroid nodules with hyperthyroidism, etc. In addition, when free triiodothyronine, free thyroid hormone and thyroid stimulating hormone are elevated, central hyperthyroidism is considered and further tests are needed to determine whether the lesion is in the pituitary or hypothalamus. If only free triiodothyronine is elevated, and free thyroid hormone and thyroid stimulating hormone are normal, you can leave the condition untreated for a while and recheck your thyroid function in a month.