Synthroid T4 (Synthroid; generic name Levothyroxine Sodium) is a thyroid hormone. Thyroid hormones are produced and released by the thyroid gland and regulate the body’s metabolism. When the thyroid gland is underactive (hypothyroidism) it does not produce enough thyroid hormone and metabolism slows.
Cytomel (T3) does speed fat loss. As a guideline, for most 12.5 mcg/day is a conservative “supplement” sort of dosing that seems to have no detectable adverse effect on thyroid function at all. 25 mcg/day is a “supplement” sort of dosing that does have some inhibitory effect. 50 mcg/day is a reasonably conservative bb’ing sort of dose that, of course, is more inhibitory. 75 mcg/day is getting into more of a problem area; 100 mcg/day in many cases leads to loss of muscle size and strength.