A thyroid uptake scan is a diagnostic tool used to assess thyroid function. It helps healthcare providers understand how well the thyroid absorbs iodine, a substance it needs to produce hormones. This procedure is particularly useful for investigating conditions like hyperthyroidism, where the thyroid is overactive, or to characterize thyroid nodules and goiters. The scan involves administering a small, safe amount of radioactive iodine, which the thyroid naturally takes up, allowing for measurement and imaging.
The Scan’s Duration
While the actual measurements during a thyroid uptake scan are relatively brief, the entire process spans approximately 24 hours. This extended timeline is necessary to allow the thyroid gland sufficient time to absorb the radioactive iodine administered.
The process typically begins with an initial visit for the oral administration of a radioactive iodine pill. This initial step is quick, often taking only about 5 to 10 minutes. You will usually return for the first measurement approximately 4 to 6 hours after taking the dose. This measurement typically involves a short scan lasting about 15 to 30 minutes.
A second measurement is usually performed about 24 hours after the initial dose. This measurement also lasts around 15 to 30 minutes. Therefore, while the time spent at the clinic for each measurement is minimal, the overall test requires two separate visits over two days to complete the full evaluation.
Preparing for Your Scan
Your healthcare provider will provide specific instructions, which often include dietary adjustments and medication changes. You may need to follow a low-iodine diet for a period leading up to the scan, typically for one week. This involves avoiding foods rich in iodine, such as seafood, iodized salt, dairy products, and certain breads.
If you are taking thyroid hormones (like levothyroxine) or anti-thyroid medications (like methimazole or propylthiouracil), your doctor will likely instruct you to stop these several days or even weeks before the scan. It is also important to inform your provider about any other medications, vitamins, or herbal supplements you are taking, as some can interfere with iodine uptake. Additionally, you should avoid any imaging tests using iodine-based contrast material for several weeks prior to your scan.
Fasting may be necessary before taking the radioactive iodine pill, with instructions to avoid food and drink for a few hours prior to the first appointment. It is also crucial to inform your doctor if you are pregnant, suspect you might be pregnant, or are breastfeeding, as the radioactive material can affect a fetus or baby. In such cases, the scan is generally not recommended.
What Happens During the Scan
During the measurement sessions, you will lie comfortably on an examination table, often with your head tilted back to allow clear access to your neck area. A specialized device called a gamma probe or scanner will be positioned over your neck, close to your thyroid gland.
This probe does not emit radiation; instead, it detects the gamma rays emitted by the radioactive iodine that has accumulated in your thyroid. The probe measures the amount of radioactivity within your thyroid, and a computer processes this data to calculate the percentage of the radioactive iodine absorbed by the gland. You will be asked to remain still during the brief scanning periods to ensure accurate readings.
Understanding Your Results
The results of a thyroid uptake scan are expressed as a percentage, indicating how much of the radioactive iodine your thyroid gland absorbed over the measured timeframes. Normal uptake values can vary slightly between laboratories but generally fall within a specific range, such as 5% to 15% at 6 hours and 10% to 30% at 24 hours. These percentages help your doctor understand your thyroid’s activity.
A higher-than-normal uptake percentage suggests an overactive thyroid, which can be indicative of conditions like Graves’ disease or a toxic multinodular goiter. Conversely, a lower-than-normal uptake can point to an underactive thyroid or conditions such as thyroiditis, where the gland is inflamed, or it may be influenced by certain medications or iodine overload.
The thyroid uptake results are typically interpreted alongside other diagnostic information, including blood tests that measure thyroid hormone levels (like TSH, T3, and T4) and sometimes other imaging studies like an ultrasound. This comprehensive approach allows your doctor to make an accurate diagnosis and determine the most appropriate course of action, which may involve further evaluation or the initiation of treatment.