Iodine is the 53rd element on the periodic table, with this configuration of electrons in its shells.
The thyroid gland is shaped like a butterfly and is located at the base of the throat. It is a main player in the the endocrine (hormone) system. The thyroid is regulated by the pituitary gland, which is regulated by the hypothalamus. Most of the body’s iodine receptors are in the thyroid (women also have a concentration in the feminine tissues, because iodine is necessary for developing babies). [1,2]
The thyroid creates hormones that direct metabolism and growth. Thyroxine (T4) and triiodothyronine (T3) are created by combining iodine absorbed from the blood (which came from the diet) with the amino acid tyrosine. The thyroid needs 4 atoms of iodine to produce T4 and 3 atoms of iodine to produce T3.  If the body doesn’t have enough iodine atoms, it cannot produce appropriate amounts of these thyroid hormones -- which is bad news down the line!
Every cell in the body uses thyroid hormones, therefore every cell in the body depends on appropriate levels of iodine in the body for optimal functioning [2, 3], yet 95% of Americans are deficient in iodine! 
While serious disease may take years to manifest, iodine deficiency guarantees sub-optimal metabolism because iodine is foundational to proper thyroid function.
Hypothyroidism is an underactive thyroid. 
In this disorder, the thyroid does not make enough thyroid hormone. It is caused by iodine deficiency. Symptoms can include: 
- Increased cholesterol
- Hair loss
- Memory loss
- Dry skin and hair
- Weight gain
Hyperthyroidism is an overactive thyroid. 
In this condition, the thyroid makes too much thyroid hormone. Paradoxically, it can also be caused by iodine deficiency. [3, 7] Symptoms can include: 
- Weight loss
- Brittle hair
- Rapid heart rate
- Trouble sleeping
- Frequent bowel movements
- Bulging eyes
- Graves’ disease, an autoimmune disorder that attacks the thyroid
Goiter is an enlargement of the thyroid. 
Untreated thyroid disorders can lead to heart disease, mental problems, infertility, diabetes, and cancer, so it is important to supply the body with enough iodine for optimal thyroid health. [8, 13, 14]
The RDA is 150 mcg for men and 220 mcg for pregnant women, and 290 mcg for nursing mothers, but these levels are extremely low given the rampant iodine deficiency in this population. [12, 13] In fact, the recommended daily allowance for iodine given by the FDA is actually the bare minimum to prevent goiter rather than to optimize health.
In the 1920s, doctors observed a prevalence of goiters around the Great Lakes. Iodized salt was introduced as a cost effective and easily distributed means of getting iodine into the food supply.  It is not actually iodiNe with an "n" but manufactured iodiDe with a "d" that is added to iodized salt (along with a slew of potentially toxic chemicals, from anti-caking agents to aluminum derivatives).  Further, there is only about 30 mcg of iodine in the recommended daily allowance of iodized salt, and only 10% of the iodine from iodide is actually available for the body. Iodized salt is really not a reliable and healthy source of iodine. 
So what does the thyroid need?
Research shows that nascent iodine is an ideal form of this element for the body to absorb; as it is pretty much ready to go, no energy is used in converting it to a useable form.
Iodine is a key for thyroid health and therefore metabolism regulation, but this is just one of many responsibilities of iodine in the body!
Iodine is an essential nutrient for the body, and it is vital to the foundational functioning of the human organism. Please consider if you are getting enough of this nutrient! There are very few reliable and sufficient sources of iodine in the American food supply, so it is a great idea to contemplate your body’s source. While there are trace amounts of iodine in some seafoods, the only real food on earth with enough to meet the body's needs are sea vegetables. Unless you're eating uncooked, unprocessed sea vegetables on a daily basis or supplementing with a high quality iodine of the proper form, there is little to no way your body is getting enough of this foundational element.