I'm currently investigating whether I might have hypothyroidism. There are at least 7 blood tests used to diagnose this condition, but NONE of them are conclusive on their own. Most doctors are clueless about proper treatment; they'll prescribe Synthroid across the board, regardless of your symptoms.
The most reliable test might be taking your body temperature immediately upon awakening in the morning - before you even get up. Do this several days in a row; not during your menstrual cycle if you're a woman. If your temperature is consistently below 97.8F, you probably have at least an under-functioning thyroid.
The most common cause may be Hashimoto's, a auto-immune disease exacerbated by gluten intolerance. Hashimoto's attacks the thyroid gland. Exposure to radiation and environmental toxins are also prime suspects.
Avoid polyunsaturated fats, especially soy products of any kind. Certain vegetables like raw broccoli and cabbage should be avoided as well.
Thyroid hormones are a last resort. Once you start on hormones, you're on them for life - even the natural kind. And you may still have symptoms because they don't treat the underlying condition.
Fluoride in water is a problem. Environmental pollutants and hormone-like chemicals in non organic produce also contribute to thyroid problems. Exercise promotes thyroid function.
I have read testimonials of people who cured themselves by eating coconut oil. Iodine supplements may help IF YOU DON'T HAVE HASHIMOTO'S. If you do, iodine will make it worse. It takes a thyroid antibody blood test to detect Hashimoto's. Don't take iodine on your own.
Seaweed is a good source of iodine, but about half of the seaweed products on the shelf are contaminated with bromine, which negates the iodine. I'd stick to organic seaweed products.
Coconut Oil: Thyroid and Weight Loss Aid, or Just Another Fat?