Kelp (and Iodine)

Kelp is a good source of several vitamins and minerals, including selenium, some of the B vitamins, beta-carotene and iron; but its best-known use is as a source of iodine (strictly, iodide).
Iodine is used by the body for one thing and one thing only; production of thyroid hormones, including thyroxin (there are at least three others). Therefore, lack of iodine leads to symptoms associated with hypothyroidism, which are: Fatigue; weakness; weight gain or increased difficulty losing weight; coarse, dry hair; dry, rough pale skin; hair loss; cold intolerance (can't tolerate the cold like those around you); muscle cramps and frequent muscle aches; constipation; depression; irritability; memory loss; abnormal menstrual cycles; decreased libido; susceptibility to infections.
These symptoms arise because the thyroid is the "master gland" controlling the general rate of metabolism, and thus lack of iodine leads to a generally reduced metabolic rate.
Iodine is a nutrient needed in rather small amounts (typically 150 micrograms per day for an adult) and the requirement doesn't vary much between people or because of circumstances, unlike some other nutrients.
The best natural sources of iodine are seafood and iodised table salt (or seasalt; the iodine occurs naturally in this). Other sources include milk and milk products, because iodine compounds are used to clean dairy equipment and the cows' udders, and the iodine gets into the milk that way. Another source of iodine is seaweed, but seaweed is not commonly eaten, at least in the UK. If you are a lover of laverbread then you will probably be getting enough iodine. Seaweeds, including kelp, are a good source of iodine because they concentrate the iodine, naturally present in seawater, in their tissues.
Iodine deficiency used to be very common in districts a long way from the sea, because the main source of iodine in the soil is a gas (iodomethane) produced in trace amounts from rotting seaweed. This substance is the main cause of the "ozone" smell associated with the seaside. Unfortunately, this iodine-containing gas is highly unstable and doesn't travel far from the sea.
Iodine deficiency can also be caused by substances in food called goitrogens. These are found in uncooked soya products such as tofu, and also in uncooked cruciferous vegetables such as cabbage; thus if you eat very large amounts of coleslaw you may end up deficient in iodine.