Triiodothyronine is secreted by the thyroid gland and low levels may be due to hypothyroidism due to various reasons. The causes of low triiodothyronine are hypothyroidism, as follows: 1. The thyroid gland itself: mainly due to the destruction of the thyroid gland itself for various reasons, hypothyroidism of the gland, and insufficient raw materials needed for hormone secretion. 2. Insufficient thyrotropin: thyroid secretion is also governed by the thyrotropin-releasing hormone secreted by the pituitary gland. When the pituitary gland does not secrete enough thyrotropin, it will also lead to thyroid insufficiency and a decrease in the secretion of hormones. Patients with abnormalities of triiodothyronine are advised to seek prompt medical attention and to actively search for the cause of the disease under the guidance of a specialist.