What is T3? It's an important thyroid hormone, known as T3, though its longer medical term is triiodothyronine. Most doctors do not really check this hormone with patients, yet it is the most metabolically active hormone in your cells. T3 is mainly responsible for creating energy in your cells, as well as your metabolism. If you suffer from things like fatigue, easy weight gain, chilliness, thinning hair, dry skin, poor memory – it’s important to have your T3 levels checked. More specifically it’s your free T3 that’s your active form of T3 hormone. For patients who are low or sub-optimal, you can use supplements like selenium, or iodide can be helpful. For my patients who are overtly deficient and need more support, we actually prescribe to them compounded natural or bioidentical T3 therapy, which is very, very beneficial.