Radioactive iodine therapy may be used in the treatment of the following conditions:
Thyroid cancer
Hyperthyroidism (an overactive thyroid gland)
It can also be rarely used to treat the following conditions:
Prostate cancer in men
Ocular melanoma or intraocular melanoma (a type of eye cancer)
Cervical or uterine cancer in women
Neuroblastoma (a rare type of childhood tumor)
Neuroendocrine tumors, like phaeochromocytomas
I-131 can be attached to a chemical known as MIBG (meta-iodo-benzyl-guanidine) and used in scans to diagnose the following types of tumors:
Neuroblastoma
Neuroendocrine tumors 

The following different types of diagnostic tests may be recommended before radioactive iodine therapy:

1. Physical examination: The doctor will examine the neck of the patient to feel for any changes in the thyroid gland, like the presence of a lump or nodule. The patient will also be asked about any past exposure to radiation, medical history, and if there is any family history of thyroid cancer.
2. Blood tests: Some medical conditions can be diagnosed using blood tests. Thyroid stimulating hormone (TSH) is a type of blood test performed to measure the blood levels of TSH and hormones produced by the thyroid gland.
3. Ultrasound: It is a type of imaging test that uses high-frequency sound waves to create an image of the internal organs, like the thyroid gland.
4. Other imaging tests: Imaging tests like CT scans, MRI scans, and ultrasounds may help the doctor in checking if cancer has spread beyond the thyroid gland.
5. Radioactive iodine scan: It uses radioactive iodine and a special camera to detect the thyroid cancer cells in the body.
6. Fine-needle aspiration biopsy: A long, thin needle is inserted through the skin into the thyroid nodule to remove some cells from the thyroid gland. The sample obtained is sent to the laboratory for analysis.
7. Genetic testing: Certain types of thyroid cancers caused by inherited genes could be detected using genetic testing.

The most common types of thyroid cancer, namely papillary and follicular thyroid cancers, can normally be treated using large doses of I-131.
The therapy is usually provided after the surgical removal of the thyroid gland to destroy any remaining thyroid tissue.
If the patient is getting I-131 in the pill form, one to three pills are given, based on the dose recommended by the doctor.
If the patient is getting I-131 in liquid form, the patient needs to drink about 5 ml of the liquid out of a vial through a straw. The liquid is usually tasteless and flavorless.
A tracer dose of I-131 may be used to track any remaining thyroid tissue or cancer that could have spread to other body parts. These tests will show if the iodine is concentrated in the areas containing thyroid cancer, and if higher doses of I-131 are needed for destroying the tumor.
The doctor will monitor the radiation levels right after treatment using a hand-held device.

Risks:
Radioactive iodine therapy may be associated with the following side effects:
Nausea
Vomiting 
Feeling flushed
Tenderness in the neck region
Swelling in the neck region
Dry mouth
Changes in taste
Salivary gland inflammation 
Gastritis (inflammation of the gastrointestinal tract)
Dry eyes
Requirement of hormone replacement medications due to low or absent levels of the thyroid hormone
Increased risk of developing leukemia (a type of blood cancer), stomach cancer, and salivary gland cancer
Infertility and low sperm count in men for up to two years following treatment
Irregular periods in women for up to one year after treatment 

The patient is given a sugar-free chewing gum or hard candy to suck on after treatment to prevent dry mouth.
The patient is recommended to avoid wearing contact lenses for some days or weeks after the procedure.
A body scan may be recommended to check if there are any remaining cancer cells in the patient’s body after giving I-131.
The radioactive iodine will pass out of the patient’s body in the form of urine and saliva.
The patient’s body will emit radiation after treatment and therefore, the patient needs to stay in the hospital for three to five days after the procedure to prevent radiation exposure to other people.
The patient should follow these instructions for approximately three days after treatment to prevent radiation exposure to others:
Limit or avoid going to public places
Drink lots of fluids
Avoid using public transport
Avoid sharing of food or utensils
Avoid preparing food for others
Sit down during urination and flush the toilet two to three times after using it
The patient should follow the following instructions for about five or more days after treatment:
Stay at least six feet away from pregnant women and small children
Avoid going to work
Sleep in a separate bed from your partner, infant, or children
A blood test is recommended after every six to twelve months after treatment to check the levels of the thyroid hormones.
If the thyroid gland becomes underactive following treatment, thyroid hormone supplement pills are recommended for the rest of the patient’s life.
Regular follow-ups are recommended to check for the success of treatment and if there is any recurrence of cancer.