It depends. If the syntroid and cytomel were giving you the same ratios of T4 and T3 as the comparative Armour.. then only the synthesis methods and inert ingredients should cause discrepancies in performance.
Humm.. searching... Synthroid
(levothyroxine sodium tablets, USP) contains synthetic crystalline L-3,3',5,5'-tetraiodothyronine sodium salts [levothyroxine (T4) sodium]. This synthetic T4 is identical to that produced in the human thyroid gland. So it should not have any allergic reactions due to host production. Cytomel
(liothyronine sodium) Tablets contain liothyronine (L-triiodothyronine or LT3), a synthetic form of a natural thyroid hormone, and is available as the sodium salt. Once again this is what our gland produces, but is not from animal preparations. Armour Thyroid
Generic name: Natural thyroid hormones TC and TD. This is a naturally prepared T4/T3 drug.
So how do they do it?
Thyroid hormone preparations belong to two categories: (1) natural hormonal preparations derived from animal thyroid (Armour), and (2) synthetic preparations(Synthroid and Cytomel). Natural preparations include desiccated thyroid and thyroglobulin. Desiccated thyroid is derived from domesticated animals that are used for food by man (either beef or hog thyroid), and thyroglobulin is derived from thyroid glands of the hog. This is the scientific spill saying we take the thyroids out of the cattle and hogs in our food processing plants, chop them up, and extract the T3 and T4 we need. Yeah, ick.
Here in the US we have the United States Pharmacopeia (USP). The USP has standardized the total iodine content of natural preparations. This is because iodine can cause added thyroid issues. Thyroid USP contains not less than (NLT) 0.17 percent and not more than (NMT) 0.23 percent iodine [in otherwords the amount of iodine demeaned safe for the average human], and thyroglobulin contains not less than (NLT) 0.7 percent of organically bound iodine[same as above.. safe accepted average]. Iodine content is only an indirect indicator of true hormonal biologic activity, but monitored all the same.
This may have been WTMI. But I like to be thorough. So it should act the same as long as the ratios are the same. The separate synthetic hormones will allow for more variation in your T3 and T4 supplementation and less chance of interaction with the inert host properties present in the Armour.