T3 is Triiodothyronine which is a thyroid hormone. It has an impact in every physical aspect of human body like body growth, muscle development, metabolism etc. In case of a steroid cycle, T3 is used as an effective PCT once the cycle is completed. And it is often used during the cycle as a fat cutting agent.
The recommended dosage of this PCT is 0.25mg to 1mg per day. Advice of a doctor is always recommended.