(1)T4/L-Thyroxine is pro-hormone, which your body converts into T3 at necessary rate only and thus it could be considered as much milder and safer drug than T3.

(2)L-Thyroxine is an excellent fat burner since your metabolism is greatly increased while being on it. You can afford to be a little sloppier on pre-contest dieting since it will still burn fat when you are taking in a lot of calories since your metabolism is going haywire.

(3)L-Thyroxine is widely used by bodybuilders and fitness addicts on pre-competition cycle in order to get relief, also used by overweighed people to reduce weight. It increases metabolism and produces general stimulating effects. However, this substance should be used wisely.

Levothyroxine or L-thyroxine is a synthetic thyroid hormonethat is chemically identical to thyroxine (T4), which is naturally secreted by the follicular cells of the thyroid gland. 

Levothyroxine or L-thyroxine is used to treatthyroid hormone deficiency, and occasionally to prevent the recurrence of thyroid cancer. 

Like its naturally secreted counterpart, levothyroxine is a chiral compound in the L-form. 

The related drug dextrothyroxine (D-thyroxine) was used in the past as a treatment for hypercholesterolemia(elevated cholesterol levels) but was withdrawn due to cardiac side effects.