What is the significance of radioactive iodine scan in various throid diseases like myxedema, hashimoto's thyroiditis and graves disease?

Hot vs. cold nodule. Iodine uptake by thyroid tissue depends on functioning cells. The most common use for thyroid scan is to assess if a nodule is functional or not. A functional/hot nodule may cause hyperthyroidism but is not likely to be malignant. See this site for more info. http://www.nlm.nih.gov/medlineplus/ency/article/003829.htm.