A thyroid uptake study is a radiographic study that is used to measure the distribution of a radioactive isotope in the body. Radioactive isotopes are special forms of elements that undergo a process called radioactive decay that involves a change from a higher energy state to a lower energy state. As radioactive isotopes undergo this change, they release small bursts of energy in the form of radiation. The tissue that makes up the thyroid gland is unique in that it is able to take up and trap radioactive isotopes called tracers. Different tracers may be ingested and absorbed through the gastrointestinal tract or injected directly into the bloodstream. Once a tracer has been taken up by the tissue that makes up the thyroid gland, a special instrument can be used to measure the energy released by the radioactive isotope. The radioactive isotope that is most commonly used to perform thyroid uptake studies is called 123-Iodine.
What information can it provide?
A thyroid uptake study can provide an estimate of the thyroid gland's level of activity. The amount of tracer taken up in the course of a thyroid uptake study reflects the amount of thyroid hormone produced and secreted by the tissue that makes up the thyroid gland. When 123-Iodine is used as a tracer, it is taken up and trapped through the same pathways that process the nonradioactive iodine circulating in the bloodstream that is used to produce thyroid hormone. If a thyroid uptake study shows that a patient's thyroid gland takes up an increased amount of tracer, it may be interpreted as demonstrating overactivity consistent with hyperthyroidism. If a thyroid uptake study reveals that a patient's thyroid gland takes up little or no tracer, it may be interpreted as demonstrating decreased activity that may be consistent with other causes of thyrotoxicosis. Thyroid uptake studies only provide useful information when they are used to evaluate patients who have been diagnosed with thyrotoxicosis.
How is it performed?
Most thyroid uptake studies are performed by radiologists or radiology technicians who take measurements that are interpreted by radiologists. Before a thyroid uptake study is performed, it is important to make sure that the patient undergoing the study is not taking any medications that contain iodine. It is also important to make sure that the patient has not undergone any recent radiographic studies using intravenous dyes that contain iodine. In either situation, previous exposure to iodine may block entry of radioactive tracer into the thyroid gland, making it difficult to obtain accurate measurements. If the patient is taking either methimazole or propylthiouracil for treatment of hyperthyroidism, the medication may need to be held for a few days before a thyroid uptake study is performed. If the patient is a woman of childbearing age, it is important to make sure that there is no chance that she may be pregnant before performing a thyroid uptake study.
When a thyroid uptake study is performed using 123-Iodine, the patient is usually told to begin fasting after midnight the evening before the study begins. Once the patient has reported for the study, the amount of radioactivity present in the dose of 123-Iodine to be used as a tracer is carefully measured. After the amount of radioactivity has been documented, the patient swallows the tracer in the form of a liquid or capsule. The first uptake measurement is usually taken about 4-6 hours after the tracer is swallowed. A special instrument that measures radioactivity is held up against the neck to count the energy released by any tracer taken up by the thyroid gland. The same instrument is then held up against another part of the body to count the background level of energy released by other tissues. This background level is subtracted from the thyroid level to provide an estimate of the amount of radioactivity present in the tracer taken up by the thyroid gland. This amount is compared to the total amount present in the dose of tracer to determine the proportion that has been taken up by the thyroid gland. This process is usually repeated about 24 hours after the tracer is swallowed to obtain a second uptake measurement. Thyroid uptake study results are usually reported as percentages measured at 4-6 hours and at 24 hours.