According to ray peat who has studied the subject for decades the biggest impactors on thyroid are PUFAs, stress and certain amino acids(methinone, trytophan, cysteine). Estrogenic foods like soy and certain vegetables are problematic as well. Not getting enough nutrition is very anti-thyroid too.

Ray also thinks that burning free fatty acids is very stressful therefore anti-thyroid so he recommends enough sugar to keep liver glycogen full, sugar in my experience has the biggest impact on thyroid as its very pro-thyroid.

A very easy way to test for hypothyroidism is body temp, you want it to be ~97.7 upon waking and 98.6 after you eat your first meal. You want your pulse around 80-90 bpms as well, high pulse+high temp could indicate adrenal stress. You can use these numbers in combination with diet to see what lowers and raises your thyroid.

The diet with the largest impact on the thyroid would be an iodine-deficient diet.

-Steve

I don't know if particular foods cause hypothyroidism, but I do have hypo and based on research I've done, I stay away from cruciferous veggies -- spinach, cabbage, brussels sprouts, broccoli, cauliflower, etc. Apparently they have a negative effect on thyroid function when eaten raw, but cooked they're fine. I don't think that eating these things raw has any effect on people with normal-functioning thyroids.

@ Cliff:

We're all dying of something. I realize there are more indications of low thyroid than the few I mentioned. I agree that low body temp can be a sign of it but it is by no means definitive when taken in isolation. Other health conditions, hormonal status, seasons,sex, and emotional state amont other factors can also affect body temp. If you choose to believe that everyone with less than the so-called normal body temp at all times is hypothyroid, that's your choice. I disagree. Here's some interesting info on the so-called normal body temp and factors affecting it:

http://www.anaturalhealingcenter.com/documents/Thorne/articles/bodytemp.pdf

http://www.anaturalhealingcenter.com/documents/Thorne/articles/BodyTemp12-1.pdf

Getting back to the actual original question which had to do with diet and it's impact on thyroid -- from my own personal experience as someone with hypothyroidism who takes medication and who gets labs taken regularly to monitor thyroid levels and antibodies -- I think each person's susceptibility to goitregens and to iodine vary. No difference in how I feel from eating lots of steamed goitrogens, tea (all varieties, cocoa, raw nuts in general. Foods that have made a difference -- a negative one -- are soy, dairy, wheat, raw walnuts in excess (I love them but have to eat these in moderation -- raw almonds on the other hand, also a goitrogen don't have the same impact) and too much iodine. I love seaweed but overdoing kelp or Lugol's has a negative impact. I think most important universal diet tip for thyroid health is adequate protein -- not high --- but adequate from a variety of organic sources -- seafood, pastured eggs, grassfed animal flesh and organs. For autoimmune (Hashis/Graves) or overweight non Hashi hypos, avoiding wheat and legumes seems helpful. From the many women I know with hypothyroidism who are also overweight, they appear to all be addicted to wheat. Just a personal observation for what it's worth.

Here's my anecdotal evidence on the diet/hypothyroid connection:

In 2005, I retired from the military, a healthy 39 yo male, 5'11, 200lbs. On no medicine. By 2007 I was 240lbs, on meds for high BP, high trigs, hypothyroid and gout. My diet was total crap and I didn't exercise. I was stressed out from moves and career changes, I had sleep apnea and just started using a CPAP. In 2010, doc said I was diabetic and needed more meds. I said "NO WAY". I found Mark's Daily Apple and the Primal Blueprint exactly 1 year ago today. Full on paleo/primal by Feb 2011. By May, I was down to 185--my bp, trigs, gout all cured--off those meds. Doc said "Synthroid is for life, my boy!"

In 2006, my TSH was 40. They treated me with Synthroid in varying doses until it got down to 2. After I started paleo eating, my TSH was always .55 (low end of normal). Doc said "this is good, it's because of the Synthroid--not your silly new diet".

June 2011 I quit taking my Synthroid. I had my TSH tested in Sept and it was 2.55 (mid-range normal). Tested it two weeks ago and it was 2.00.

Now, I was never diagnosed with Hashimotos or tested for Thyroid antibodies. When I would ask the doc why my thyroid wasn't working, he'd say, "sometimes they just don't".

I think that once I got my weight under control, fixed my sleep, and reduced my stress--my TSH returned to normal. I don't think there was ever anything wrong with my thyroid, just a bunch of metabolic derangement that caused high TSH.

Avoid toxic foods (wheat, soy etc) and listen to Ray Peat (although I'm still not convinced of fructose over starch...). :-)

For the last three years, I have suffered symptoms of hypothyroidism then someone recommended natural thyroid supplements and I tried it as soon as possible. After few weeks I felt the incredible changes in my body. I’m so happy.