Thyroid Scintigraphy

Introduction

Thyroid scintigraphy is well established in nuclear medicine and even though the advent of ultrasonography and other techniques has somewhat reduced its role in diagnosis it still remains a reliable diagnostic tool for evaluating patients with thyroid problems. Radiopharmaceuticals used to image the thyroid are iodine-131, iodine-123 and technetium-99m. Iodine, which is trapped by the thyroid gland, is an obvious choice for imaging because it is a precursor in thyroid hormone synthesis and as a result undergoes organification and binding to thyroglobulin. Technetium does not undergo organification but it is trapped and concentrated by the thyroid gland thus making imaging possible. Each radiopharmaceutical has its advantages and limitations and there is no one superior agent for imaging.

Clinical Applications

Clinically, thyroid scintigraphy serves as the next step after physical examination with confusing findings and in the management of patients who have undergone treatment for thyroid cancers.

Goitre

A number of conditions including Graves' disease and Plummers' disease are associated with goitre. Thyroid scintigraphy after physical examination can either determine which condition is the cause or rule out conditions as the causes. Figure 4.1 is a technetium scintigram of a patient presented with goitre. Abnormally high uniform uptake is an indication of diffuse hyperplasia and Graves's disease.

Nodules

Nodules are classified as either hot (functioning) or cold (non-functioning). The terms hot and cold simply refer to their appearance on a scintigram (hot spot - photon rich, high concentration, cold spot - photon deficient, low concentration). It has been reported that 5-40% of cold nodules are malignant. The reason for such a large range is the result of certain factors. For example the incidence of benign nodules increases with age thus a younger patient is of greater concern than an older on. Also nodules are more common in women than men. A young male presented with a solitary cold nodule is of greater concern than an old female with the same symptoms. Malignancy cannot be confirmed or ruled out by scintigraphic findings; instead scintigraphy is part of the clinical workup of patients presented with solitary cold nodules.
Less than 1% of solitary hot nodules are malignant. Hot nodules are more indicative of hyperthyroidism and the diseases associated with it. There however exist an important finding in thyroid scintigraphy; some solitary hot nodules found on technetium scans are presented as cold nodules on radioiodine scans. This discordance between technetium and iodine is one of the drawbacks of technetium thyroid scintigraphy and consequently a thyroid with a solitary hot nodule should be re-imaged with iodine before confirmating the nodule as functioning.

Radioiodine or Technetium

The table above compares some of the characteristics of iodine-131, iodine-123 and technetium-99m. Technetium and I-123 have ideal energies for scintigraphy; the energy of I-131 is too high for scintigraphy because of reduced detection efficiency. Technetium, with a half-life of 6 hours is ideal because it is readily available in nuclear medicine departments or pharmacies. I-123 has a suitable half-life for nuclear medicine but it is expensive to produce, contains long-lived impurities and is not readily available (the short half-life is actually a disadvantage because of the poor availability). The good availability and long half-life of I-131 permit transport over long distances to radiopharmacies or departments. The long half-life also makes delayed imaging at 48 and 72 hours, for equivocal findings, possible. For substernal thyroid, whole body imaging for metastases and uptake studies technetium cannot be used. This is a major drawback of technetium and requesting physician should be aware of this and also the discordance with solitary hot nodules.