Too often I encounter well-meaning pet caregivers that immediately supplement a dog’s diet with kelp (a rich source of iodine) based on assumptions of observed symptoms of thyroid dysfunction. Iodine is essential for the production of thyroid hormones T3 and T4. Diagnosing iodine deficiencies is tricky, though. In fact, iodine deficiency diagnoses are generally left to epidemiologists who infer it by the amount of iodine in the soil where people and pets live, or measure the iodine content of the urine of a population as a whole rather than an individual.
In terms of dogs, kelp supplementation will only work as intended if the dog has an iodine deficiency, which is highly unlikely. The majority of dogs that become hypothyroid suffer from inherited autoimmune thyroiditis (like Hashimoto’s lymphocytic thyroiditis in people), which has nothing to do with iodine deficiency. Further, excessive iodine supplementation can result in the overproduction of the T4 and T3 in dogs and cats, which triggers unintended cascading effects: in dogs, the immune system may wind up attacking the thyroid gland (producing excessive amounts of thyroglobulin autoantibody) which end up suppressing thyroid levels and causing the very hypothyroidism it was meant to prevent; whereas in cats, the overdosing can result in overt hyperthyroidism.
Common sense begets that you should also not give kelp to dogs that are taking thyroxine via a commercial brand name or generic synthetic thyroid product.
Observation is the first critical step; and thorough testing is ultimately the most important next second step to treat a pet or human with potential thyroid function problems.