Synthroid is the less powerful of the two most popular thyroid replacement drugs on the market. It is synthetic T4, and is actually the more prescribed thyroid medication in America, but the lesser used of the thyroid drugs which are popular with bodybuilders. If you have naturally low T3 levels, then you may be able to supplement with T4, and have it convert to T3 via your bodys natural metabolic pathways, which involves the deiodinase enzyme. However, There have been a number of studies that have shown that during reduced caloric intake, and/or when carbohydrate intake is reduced dramatically, levels of deiodinase decline, hindering the conversion of T4 to the physiologically active T3(1). So, if you are dieting (which would necessarily mean you have a reduced caloric intake and/or reduced carbohydrate levels), then you have less deiodinase enzyme (still with me?) and thus, that T4 you are taking in hopes of getting it to convert to T3, is not getting converted. This is not what we want, clearly, and is why most pre-contest dieters include Cytomel in their drug regimen instead of Synthroid. In fact, Synthroid may be particularly bad for dieters on Cyclic Ketogenic Diets.
When you earn your living off of your body, as many fitness models, models, and bodybuilders do, its just too haphazard to trust Synthroid. This is especially true if you are not monitoring your Basal Body Temperature or (preferably) shelling out the money for a thyroid function test every month.
As compared to Cytomel, Synthroid requires significantly higher doses to be effective. Most bodybuilders dont exceed 100mcgs of Cytomel during a precontest phase of dieting, but with Synthroid the doses climb significantly higher to achieve the same results. From interviews Ive done with bodybuilders who have used Synthroid, Ive heard of it being used at up to 300mcgs/day. When you compare that to the mere 25-100mcgs/day of Cytomel that bodybuilders are typically using, we have another strike against Synthroid. It isnt really economically feasible to do that much Synthroid and remain cost effective, at least when compared with Cytomel. To give you a fair estimate, you could run an effective dose of both Cytomel and Clenbuterol for the same price as an effective dose of Synthroid.
My advice? Use it if you have to, but only if thats the case, and you cant get Cytomel or Tricana. It works, and will eventually get you to the desired body temperature for optimal fat burning, but it just isnt as elegant as the less suppressive Tricana, or the more effective Cytomel.
- The effect of a low-calorie diet alone and in combination with triiodothyronine therapy on weight loss and hypophyseal thyroid function in obesity. Koppeschaar HP, Meinders AE, Schwarz F. Int J Obes 1983;7(2):123-31