It appears that the understanding of the thyroid gland and its physiologic function has escaped this discussion. I would recommend the reading of "Medical Physiology" by Arthur Guyton for a complete and accurate description of the thyroid feedback pathways.
It all starts in the brain. Thyroid Stimulating hormone-Releasing Hormone stimulates the pituitary gland to make Thyroid Stimulating Hormone, which in turn feeds to the Thyroid gland to make Thyroid hormone (T3) Tri-Iodothyronine.
T3 thyroid hormone normally synthesized and secreted by the
thyroid gland in much smaller quantities than
thyroxine (T4). Most T3 is derived from peripheral monodeiodination of T4 at the 5' position of the outer ring of the iodothyronine nucleus. The hormone finally delivered and used by the tissues is mainly T3. (simply put, the body clips off one iodine molecule from T4 to make T3)
levothyroxine; L-thyroxine; thyroxine ...
IUPAC: (2S)-2-amino-3-[4-(4-hydroxy-3,5-diiodophenoxy)-3,5-
diiodophenyl]propanoic acid
MW: 776.870020 g/mol | MF: C15H11I4NO4
Synthroid is generally T4(levothyroxine) which is chemically exactly the same as human thyroxine. Synthroid is a replacement for a hormone that is normally produced by your thyroid gland to regulate the body's energy and metabolism. Synthroid is given when the thyroid does not produce enough of this hormone on its own.
Synthroid treats hypothyroidism (low thyroid hormone). Synthroid is also used to treat or prevent goiter (enlarged thyroid gland), which can be caused by hormone imbalances, radiation treatment, surgery, or cancer. It should not be used to treat obesity or weight problems.
Read more:
http://www.drugs.com/synthroid.html#ixzz117rQeqcl
The T3 after this deiodination feeds back to the pituitary gland to regulate the production of TSH.
Simply put, this feedback loop is the process of control.
T4 being converted to T3 is the step that is often the problem because it occurs outside the thyroid gland. However if the thyroid gland fails for one reason or another, it becomes necessary to supply the body with its product.
This particular area of endocrinology is frequently the least understood and therefore frequently poorly managed. There are numerous explanations for why but the bottom line is that because of the complexity of the system, many folks don't bother to learn it and be comfortable explaining it...as I am seeing here.
As mentioned earlier, iodine is in one of the most common food additives used, i.e. table salt. It was added because of the incidence of hypothyroidism in the midwestern US when it was realized that the diet was deficient in Iodine.
Rather than issue "advice" on the subject here, I suggest a thorough reading of the available and biochemically correct/accurate information available on the Internet and seek COMPETENT medical advice on your own SPECIFIC issue rather than general advice from someone who may NOT know YOUR specific problem.
No two fish in the sea are exactly the same. Furthermore, it is frankly rather insulting to listen to anyone who suggests that physicians are dishonest and ethically degenerate by getting "kickbacks from the pharmaceutical companies" or are in any way controlled by them because it just isn't the case. It is myth perpetrated by folks who have an axe to grind with doctors, and many of those folks are just frustrated people who either wanted to be a doctor or are in some way jealous of being a doctor. Becomming a doctor takes adequate intellectual capability and
a significant number of years of study and personal sacrifice that those folks who are so blatantly critical are UNWILLING or INCAPABLE of doing. Otherwise they would have become doctors themselves.
The world would be better served by not attempting to propogate myths which serve no good to the general population other than to inflate their own ego at the expense of folks in society who attempt to do good.
YMMV