You took the kelp and weighed less and felt better and then quit the kelp and you have salt cravings and gaining weight? Hmmm what do you think sounds bad to you...
you have to do things in moderation which sounds like what you were before you quit taking the kelp.
Ultimately you know how you feel and I'm sure you can modify how much or how little of kelp you take to help you feel better.
The only other thing that could be happening is that your thyroid is failing further and it's just a coincidence that at the same time you went off the kelp your thyroid got worse.
Trying the kelp again and seeing if that helps the weight gain and other symptom.
Yes, when taking certain amounts of supplements with certain amounts of medications the combination can be deadly. This is why warning are put on them now.
Feel free to discuss this with your MD, let him know how you felt while on both the kelp and thyroid medication and how you felt after stopping the kelp. If he tries to put you on more thyriod medication, he's trying to do the same thing that supplementing with kelp was doing.
Let me know what you find out.

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Hi I'm Dr. Berkebile from Johnstown, PA and in private practice. Providing Chiropractic, cold laser therapy, services similar to Physical Therapy and Rehabilitation in Johnstown PA, Nutritional and Supplement counseling, and Fitness oriented programs. As an educator and author, I've conducted workshops throughout the Northeast. I have driven Hyundai's for many years and really like their reliability.
Berkebile Chiropractic
www.dcberk.com www.johnstownchiropractor.com