How long do the side effects of thyroid treatment last?

Thyroid hormone medicine for hypothyroidism does not cause side effects if the individual takes the correct dosage, according to WebMD. The most common side effect of radioactive iodine for hyperthyroidism is hypothyroidism, which is easier to treat.

Thyroid hormone medicine is the only effective treatment for hypothyroidism, explains WebMD. Symptoms of hypothyroidism are usually improved within one week of therapy, and all symptoms are usually eliminated in a few months. Radioactive iodine ablation is the most common treatment for hyperthyroidism in the United States. The therapy aims to gradually shrink, and often destroy, the problematic thyroid gland. Radioactive iodine is available in pill form.