Radioactive Iodine Uptake
The thyroid gland is made up of a special type of cell that absorbs the iodine circulating in the body and uses it to produce thyroid hormones. Radioactive iodine uptake scans take advantage of this quality of thyroid cells and use a type of labeled iodine to pinpoint any remaining thyroid tissue in the body. These scans can be used to identify residual thyroid tissue after surgical removal of the thyroid, or cancerous tissue that has metastasized, or spread, to other organs.
How Radioactive Iodine Works
Radioactive iodine can also be used in larger doses as a treatment, as the radiation will ablate, or destroy, any remaining thyroid tissue (RAI treatment). Before a patient undergoes either a radioactive iodine uptake scan or a radioactive iodine treatment, they will be put on a low iodine diet. This page will explain why the diet is important, how it works, and how best to adhere to the diet.
Where is Iodine Found?
Iodine is found in the soil and in almost everything we eat, especially dairy products and seafood. Iodine can also come from chicken and cattle that have been fed with iron-supplemented feed.
Low Iodine Diet
In the United States, a normal diet usually contains about 150 mcg of iodine per day. Generally, the goal of a low iodine diet is to consume less than 50 mcg per day.
On a low iodine diet, foods are placed into three categories: avoid, limited, and unlimited. Foods that are high in iodine should be avoided, foods that have variable iodine content should be eaten in limited quantities, and foods with low iodine content can be eaten in unlimited quantities.
It is important to read the labels on all foods to make sure they do not contain ingredients such as iodized salt, egg yolks, and FD&C red dye #3. Non-iodized salt should be consumed instead of regular salt and can be purchased at most grocery stores.
Why is a Low Iodine Diet Important?
Prior to any form of radioactive iodine scan or treatment, patients should be placed on a low iodine diet. This diet deprives the body of iodine, and makes radio-iodine uptake more effective. When the body senses low iodine levels, the pituitary gland in the brain releases TSH (thyroid stimulating hormone), which stimulates the thyroid cells to absorb more iodine.
This diet helps to ensure that the thyroid cells will uptake the radioactive iodine that is administered for the scan or treatment, rather than any normal iodine that may be circulating in the body. The scan or treatment will only work if the thyroid cells uptake sufficient amounts of the radioactive iodine.
How Long Does This Diet Last?
The length of time that a patient must follow a low iodine diet will be determined by their doctor, and varies among different patients and treatments. Research has not determined a set minimum length of time required for the diet, but most doctors recommend starting the diet 1 week before a radioactive iodine scan and 1-2 weeks before a treatment. Clinicians also often recommend that the low iodine diet be continued for 1-2 days after radioactive iodine therapy.
Patients who have recently undergone a CT scan with contrast may be required to follow a low iodine diet for a longer period of time. This is because the contrast used for CT scanning contains high levels of iodine. In these cases, patients may need to adhere to a low iodine diet for 6 weeks or even up to several months before their iodine levels becomes low enough for a radioactive iodine scan or treatment to be successful. The contrast given for MRI scans does not contain iodine, and therefore radioactive iodine treatments are not affected by recent MRI studies. The same is true for ultrasound studies, which do not use any contrast material.
Foods to Avoid
- Iodized salt.
- Food from the sea.
- Dairy products.
- Whole eggs or egg yolks.
- Foods from commercial bakeries.
- Vitamins, meal replacements or food enriched with vitamins that may contain iodine.
- Medications that contain iodine
- Red food coloring dye #3.
Limited Foods
- Meats
- Grains & cereals
- Dried beans
- Rice
- Egg whites
- Soy
Unlimited Foods
- Fresh fruits & juices.
- Fresh vegetables.
- Nuts.
- Sugar, jelly, honey, maple syrup and unsulfured molasses.
- Black pepper and fresh or dried herbs.
- All vegetable oils.
- Salad dressings.
- Sodas and carbonated water, unless they have Red Dye #3.
- Coffee and tea, beer, wine and other alcohol.
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