It appears that you have hypothyroidism which requires you to take Synthroid (a synthetic thyroid hormone). The majority of the time in developed countries, hypothyroidism is caused by autoimmune destruction of the thyroid gland. Rarely, hypothyroidism can be caused by iodine deficiency – but this is extremely uncommon in developed countries as the salt is usually fortified with iodine.
Sea Kelp contains high levels of iodine and may be recommended by some practitioners of alternative medicine for “thyroid support.” In terms of the physiologic need for iodine, seas kelp supplementation would ONLY be necessary if the person is actually deficient in iodine. In most other cases, Sea Kelp supplementation would have no effect or even a potentially adverse effect.
Although every individual is different, the vast majority of patients prescribed Synthroid have hypothyroidism that is unrelated to iodine deficiency. Therefore, chances are, sea kelp supplementation would not be beneficial and you should continue taking Synthroid. If you do decided to take sea kelp, you should first consult the doctor who prescribed your Synthroid to determine if iodine supplementation is beneficial or safe.