Globulin from the liver is combined with iodine to make thyroglobulin in the thyroid - which is stored in the thyroid. later a total of 4 iodines are attached to this creating T4 (thyroxine). T4 is becomes the more active T3 when an iodine is removed via an enzymatic reaction catalyzed by selenium. Sometimes the wrong iodine is removed and Rt3 (reverse T3) is created which looks just like T3 on the typical blood test but which is totally inactive - but that is another story.
Before the numerous blood tests which exist today low thyroid was determined by taking ones temperature. Thyroid regulates metabolism - and is intimately involved in burning fuel (fats and sugar). When fuel burns it produces heat (in fact calorie is the chemistry term for a measure of heat - if memory serves me it is the number of joules needed to raise one liter of water one degree celsius) - when our body burns fat is creates heat. Too much heat = hyperthyroid, too little heat=hypothyroid.
So a quick and fairly accurate home test for the status of your thyroid is to take your temperature 9 times - 3 times a day for 3 days at various times (but not after drinking a cold soda or drinking hot chocolate) - finding the average and seeing if it is above, below or just at 98.6 F.
If it is too low it is highly possible you are low in thyroid...or at least active thryoid, as is the case with producing Rt3. Other symptoms of low thyroid are constipation, dry skin (which is also common just because its winter), feeling tired, elevated cholesterol.
A general boost in thyroid naturally can occur from tyrosine - which also may aid in getting a tan and increase energy (if basic biochem memory serves me) since both melanin (not melotonin) and epinephrine are created from tyrosine,
Its doubtful that anyone is deficient in iodine nowadays.
Other things that I think are more productive naturally are 7- keto DHEA (which does not convert to sex hormones the way regular DHEA does), and thyroid extracts - especially thyrotrophin PMG from Standard Process.
If someone is producing too much Rt3 than selenium may help. People with Irish or American Indian backgrounds as well as any woman that has been pregnant have a tendency to produce Rt3 as a survival mechanism to avoid starvation from burning calories too fast (American Indians and Irish have a long history of famines).
Liver detoxification is important for several reasons - so NAC to aid both phase I and Phase II detox, MSM and glucosamine sulfate are good to aid the sulfation pathway, and of course glucuronidation (Calcium D glucurate) is the failsafe workhorse of the liver detixification pathway and is aided by eating cruciferous vegetables. Keep in mind phase I detox creates numerous free radicals and so anti-oxidants are important to detoxification as are B vitamins, MG, MB, Zn.
Fish oils aid in thyroid production and also stimulate glucuronidation.
Steroid hormones are primarily and typically detoxified via sulfation - but when that pathway is overloaded glucuronidation steps in.
Excess iodine will supress thyroid. In fact potassium iodide is used in radiation emergencies (such as fallout) to supress the thyroid gland so it does not take up radioactive material, thereby poisoning the thyroid gland. Too little iodine creates a type of hypothyroidism called CRETINISM. But too much also creates a form of hypothyroidism.