You will have to have your doctors permission first to take it.
T5 is a type of thyroid hormone. There are two ways to increase the levels of all these types of hormone; you can take supplements, which are designed to ensure your body is working with the nutrients needed to ensure production of all Thyroid Hormones. Alternatively, as a last resort, your GP can give you Liothyronine sodium, available in tablets of 5, 25, 50 mcg.
Liothyronine sodium is not available without a GP's referral due to the dangers of incorrect administration, and you should avoid antibiotics unless you doctor ok's it.
There are products known as 'stacks', which sometimes calls themselves T5. These products usually contain stimulants such as caffeine, and will not increase T5 levels.
Alkelite containing supplements use natural ingredients all derived from non animal products which safely optimise thyroid hormone levels, obviously Liothyronine sodium will provide faster results, albeit not without significant dangers.