Despite the apparent connection between iodine intake and body metabolism, it does not follow that by increasing iodine intake with kelp, one will increase body metabolism. An exception is when people with goitre caused by iodine deficiency receive excessive amounts of iodine. The goitre then becomes toxic with high levels of thyroid horn-cone in the blood.
The RDI (Recommended Dietary Intake) for iodine is 150 micrograms per day. Kelp can contain levels as high as 4500 micrograms per 1 gram. Fortunately, communities who use seaweed as a food, for example in Korea and japan, do not usually develop problems with their iodine intake because of the body's ability to deal to some extent with excessive intakes. In extreme cases however, excessive kelp intake could interfere with normal thyroid function.
It should also be mentioned that seaweeds are a source of an interesting form of dietary fibre, but again, this is unlikely to contribute to weight reduction (see SOME DIET AIDS ALLOW YOU TO EAT ALL YOU WANT AND STILL LOSE WEIGHT).