T4 is a prohormone and less active when compared to T3 - this is the reason why many patients with hypothyroidism on Synthroid fail to lose the excess body
weight.
For increasing the basal metabolic rate (BMR) which is required
for weight loss, T3 is more important than T4. Higher your T3 levels, the better your BMR. Cytomel is therefore added to Synthroid to effect weight reduction. The effect of Cytomel takes a couple of weeks to set in. Cytomel can be given upto 100 mcg/day.