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Graves disease
Hyperthyroidism
Hyperthyroidism is most commonly caused by Graves' disease, an autoimmune disorder mediated by thyroid-stimulating antibodies. Clinical manifestations due to increased metabolism include weight loss, tremor, heat intolerance, palpitations, and exophthalmos.
The radiotracer uptake by this patient's hyperplastic and hyperfunctioning gland is uniform and intensely increased in the right lobe, left lobe, and isthmus. The pyramidal lobe, which normally has little or no tracer uptake, is also hyperplastic and demonstrates increased uptake. Therefore, the clinical manifestations and abnormal thyroid function tests correlate with the scintigraphic imaging findings.
Technetium-99m-pertechnetate (TcO4) is an efficient radiotracer for thyroid scanning. It is trapped and concentrated, but not organified, by the thyroid. The advantages of TcO4 are several: it is readily available from a molybdenum-99/Tc-99m generator, it is taken up by the thyroid in 20 minutes, and it has a short physical half-life of 6 hours. This radiotracer provides a lower radiation dose per unit administered than any of the radioiodines (I-131 and I-123) that are used for thyroid imaging. A disadvantage of TcO4 is its lower target-to-background ratio than the radioiodines. For most patients, the advantages of TcO4 outweigh its disadvantages.
Preparation of the patient prior to thyroid scanning is important. All medications that may interfere with the thyroid's uptake of the radiotracer are discontinued. Female patients must be asked whether they are pregnant or breast feeding. Thyroid scanning is contraindicated in pregnancy, especially in the first 12 weeks, because it causes suppression of the fetal thyroid tissue. The radiotracer is secreted in human breast milk, so breast feeding must be discontinued for 24 hours after the scan. Following patient preparation, TcO4 is injected intravenously using a dose of 10 mCi. The patient is placed supine with the neck extended. A gamma camera is used with a 3 to 6 mm aperture pinhole collimator (for magnification) and a 20% window centered at 140 KeV. Twenty minutes after injection, images are obtained in the anterior, right anterior oblique, left anterior oblique, and bird's eye projections for 200K to 250K counts. The bird's eye view gives an overall view of the thyroid (from the chin to the manubrium) in relation to its surrounding structures. This view also allows comparison of tracer uptake in the salivary glands to the thyroid gland. In this patient, uptake in the thyroid exceeded uptake in the salivary glands, which is consistent with hyperthyroidism. Thyroid scintigraphy with TcO4 provides practical information about the functional status of the thyroid gland.
Hyperthyroidism is most commonly caused by Graves' disease, an autoimmune disorder mediated by thyroid-stimulating antibodies. Clinical manifestations due to increased metabolism include weight loss, tremor, heat intolerance, palpitations, and exophthalmos.
The radiotracer uptake by this patient's hyperplastic and hyperfunctioning gland is uniform and intensely increased in the right lobe, left lobe, and isthmus. The pyramidal lobe, which normally has little or no tracer uptake, is also hyperplastic and demonstrates increased uptake. Therefore, the clinical manifestations and abnormal thyroid function tests correlate with the scintigraphic imaging findings.
Technetium-99m-pertechnetate (TcO4) is an efficient radiotracer for thyroid scanning. It is trapped and concentrated, but not organified, by the thyroid. The advantages of TcO4 are several: it is readily available from a molybdenum-99/Tc-99m generator, it is taken up by the thyroid in 20 minutes, and it has a short physical half-life of 6 hours. This radiotracer provides a lower radiation dose per unit administered than any of the radioiodines (I-131 and I-123) that are used for thyroid imaging. A disadvantage of TcO4 is its lower target-to-background ratio than the radioiodines. For most patients, the advantages of TcO4 outweigh its disadvantages.
Preparation of the patient prior to thyroid scanning is important. All medications that may interfere with the thyroid's uptake of the radiotracer are discontinued. Female patients must be asked whether they are pregnant or breast feeding. Thyroid scanning is contraindicated in pregnancy, especially in the first 12 weeks, because it causes suppression of the fetal thyroid tissue. The radiotracer is secreted in human breast milk, so breast feeding must be discontinued for 24 hours after the scan. Following patient preparation, TcO4 is injected intravenously using a dose of 10 mCi. The patient is placed supine with the neck extended. A gamma camera is used with a 3 to 6 mm aperture pinhole collimator (for magnification) and a 20% window centered at 140 KeV. Twenty minutes after injection, images are obtained in the anterior, right anterior oblique, left anterior oblique, and bird's eye projections for 200K to 250K counts. The bird's eye view gives an overall view of the thyroid (from the chin to the manubrium) in relation to its surrounding structures. This view also allows comparison of tracer uptake in the salivary glands to the thyroid gland. In this patient, uptake in the thyroid exceeded uptake in the salivary glands, which is consistent with hyperthyroidism. Thyroid scintigraphy with TcO4 provides practical information about the functional status of the thyroid gland.
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