If you've be suffering with Arthritis since childhood I suspect it is some autoimmune-driven Arthritis rather than plain vanilla osteoarthritis. Autoimmune diseases are frequently managed nicely with strict dieting. Gluten sensitivity and dairy sensitivity are both very common. Other food sensitivities might exist as well (eg, to peanuts, soy, eggs). Various supplements can be helpful but cannot compete with your own body firing up antibodies to tackle gluten (, for example). So I strongly recommend exploring diets such as Paleo and SCD. You can learn about these diets on the internet. For these diets to work you must adhere to them 100% and keep at it for at least 90 days. If effective you'll probably have to be on the diet for life.
If you have osteoarthritis you might be able to get away with a less restrictive diet but you'd want to eliminate inflammatory foods and consume more anti-inflammatory foods (eg, stuff with omega-3 fatty acids). More specific dietary suggestions and supplements suggestions for tackling osteoarthritis are on the internet.
Having said all this, I don't see anything wrong with you starting Iodine supplementation. Correcting an Iodine deficiency can do wonderful things. But its effects are blunted if you are on a diet unhealthy for you.