The Thyroid Uptake Scan (TUS) is a nuclear medicine procedure designed to assess the functional capacity of the thyroid gland. This test measures the percentage of a small, orally administered dose of radioactive iodine that the gland absorbs over a defined period. Since the thyroid naturally uses iodine to produce hormones, this absorption measurement directly reflects how active the gland is at capturing and utilizing iodine. The TUS primarily helps physicians differentiate between various causes of hyperthyroidism, or an overactive thyroid, and evaluate the functional status of specific thyroid nodules.
Essential Preparation Steps
Preparation for the TUS focuses on ensuring the thyroid gland is ready to absorb the radioactive iodine dose effectively. Because the test measures the gland’s ability to absorb iodine, anything that saturates the body’s iodine stores must be cleared out beforehand. This preparatory phase can take several weeks.
One preparation step is temporarily stopping thyroid-related medications, as these can alter the test results. Thyroid hormone replacement drugs, such as levothyroxine, must be held for about four weeks prior to the scan. Anti-thyroid medications, like methimazole or propylthiouracil, are usually stopped for a shorter period, often between four days to one week.
Patients must also follow a low-iodine diet for one to two weeks before the procedure. This restriction involves avoiding foods and substances rich in iodine, such as seafood, kelp, iodized salt, and certain vitamin supplements. The goal is to deplete the body’s non-radioactive iodine reserves, maximizing the thyroid’s uptake. Intravenous iodine contrast dyes, often used in CT scans, must be avoided for six weeks or more before the TUS, as they can completely block the thyroid’s absorption.
The Scan Procedure Day by Day
The Thyroid Uptake Scan is a non-invasive procedure that spans two days to allow time for the radioactive material to be absorbed. Day one begins with the administration of a small, tracer dose of radioactive iodine, most commonly Iodine-123 (I-123), swallowed as a capsule or liquid. This dose poses a minimal radiation risk.
Patients return to the nuclear medicine department for the first measurement, typically four to six hours later. During this visit, a specialized instrument called a gamma probe or scintillation counter is positioned near the neck. This device measures the amount of radioactivity the thyroid has absorbed from the initial dose.
The patient returns for the final measurement approximately 24 hours after the initial dose. The gamma probe is used again to calculate the radioactive count over the thyroid area, comparing the result to the original administered dose. The difference determines the final uptake percentage. This two-part measurement is necessary because different thyroid conditions show distinct uptake patterns at the 4-hour versus the 24-hour mark.
Interpreting Your Uptake Percentage
The final result of the TUS is a percentage indicating how efficiently the thyroid gland captured the radioactive iodine. This measurement helps classify the underlying cause of a thyroid disorder. A typical normal range for the 24-hour uptake measurement falls between 10% and 35%, though laboratory ranges can vary.
A high uptake percentage suggests the thyroid gland is highly active in capturing iodine from the bloodstream, which is characteristic of hyperthyroidism. This is often seen in Graves’ disease, where the entire gland is diffusely overactive, or in toxic multinodular goiter, where specific nodules are hyper-functioning. A high uptake confirms that the gland itself is the source of excess hormone production.
Conversely, a low uptake percentage, often below the normal range, indicates the thyroid is not actively absorbing the iodine dose. This outcome suggests that hyperthyroidism is likely not caused by an overactive gland but by a different mechanism. A low result is frequently observed in thyroiditis, an inflammation of the gland that causes stored hormone to leak out, or when the body’s iodine stores are saturated from external sources. The TUS result, when analyzed alongside blood tests for Thyroid Stimulating Hormone (TSH) and thyroid hormones (T4), provides the data a physician needs to confirm a diagnosis and determine the appropriate treatment strategy.