I'd like to try to clear up the question of why synthetic thyroid
replacement is prescribed over natural animal-derived thyroid hormones for
the treatment of certain thyroid disorders. Regarding my credentials, I'm a
former RN (BSN), taught human anatomy and physiology at the college level
and am currently a professional biologist (MS degree).

First, synthetic (or manufactured) medications (Synthroid, Levoxyl, and
Levothroid) are the drugs of choice in the treatment of thyroid disorders
because they only contain thyroxine (T4). Animal-derived thyroid
medications contain not only thyroxine, but also triiodothyronine (T3). In
the body, T3 is naturally produced from T4. Taking in T3 in addition to T4
(which is changed into T3) can result in very high blood levels of T3.
These abnormally high T3 blood levels can lead to rapid pulse and increased
workload on the heart - which can be a serious problem to anyone with
underlying (or undiagnosed) heart disease. It also becomes difficult to
determine the cause of symptoms which seem to indicate a change in the
thyroid disorder (in other words, is the complaint of a rapid pulse and
nervousness due to a change in thyroid function, or an excess of T3 in the
blood from the animal-derived thyroid medication the patient was taking?)
Therefore, a manufactured preparation containing only T4 is the preferred
treatment.

Brand names of thyroid hormones are preferred over generics because of
their consistently reliable dosages and potency. Studies conducted in the
United States have shown that this is not always case for generic T4
preparations (and even more so for animal-derived thyroid replacements).

Finally, a word of warning regarding some medications found in health food
stores. Preparations which are labeled as "thyroid support" or "aid to
weakened thyroid function" often contain animal-derived thyroid extracts
(hormones). If you are currently taking a thyroid replacement hormone or
have been diagnosed with a thyroid problem such as Graves disease, avoid
these preparations!  Always check with your doctor, first, before taking
any new medication or herbal remedy to be sure that it will not interfere
with your thyroid treatment.

Hope this helps!