Soy is a well-known goitrogen - thyroid inhibitor.
Soybeans were first grown as fodder for hogs. They were cheap, and they fattened them up better than most other things precisely because they shut down the animals' thyroid glands. When consumers started wanting leaner cuts of meat, pork producers could no longer feed soybeans to the hogs, and the bottom fell out of the soybean market.
Voilá! ... to keep bean farmers from going broke, the campaign to sell soybeans as "health food" for people was born! Now that's some slick marketing...
Illmakeit... I'm sorry. I don't know the answer to your all questions. I haven't researched it that deeply. I do know that soy does inhibit the manufacture of hormone by the gland, but that's all I know.
Update: I did a search and found the nih site had an article 3/2006 about the effects of soy protein and soybean isoflavones on thyroid function which included healthy adults and hypothyroid patients.
[url]http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=A bstract&list_uids=16571087&query_hl=4&it ool=pubmed_docsum[/url]
They found little evidence that in people having normal thyroid gland function, iodine abundance individuals, soy foods, or isoflavones adversely affect thyroid function. Remember, this is WITH plenty of iodine.
HOWEVER, some evidence suggests that soy foods, by inhibiting absorption, may increase the dose of thyroid hormone required by hypothyroid patients.
I am sure there is more info somewhere - just don't have the time to research, so I'll continue to stay away from soy.
"I'll make it"
Jewel TT 1/26/06 -Levothyroxine 75-88