Radionuclide Imaging

Thyroid Factor

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In regions with adequate dietary iodine intake, the 24-hour uptake of oral radioiodine is 10-35%. The trapping mechanism is the sodium-iodide symporter (NIS), which is regulated by thyrotropin (TSH) [3]. There are more than 20 radionuclides of iodine, but only 123I and 131I are in widespread clinical use. 123I has a relatively short half-life and emits only y photons and is used for thyroid uptake measurements and scintigraphy. In contrast, 131I has a half-life of 8 days and emits p particles as well as high-energy y photons. 131I is therefore suited for therapy, but the y photons can be imaged, which explains why 131I is used for diagnostic and post-treatment whole-body scanning in patients with thyroid cancer. The use of 131I for routine thyroid scintigraphy is discouraged because the radiation dose is about 100 times greater than that of 123I. Based on low cost, availability and an even lower radiation dose, technetium (99mTc) pertecnetate is an attractive alternative to 123I and consequently recommended for routine thyroid imaging by authorities in most European countries. 99mTc is administered intravenously, and uptake and scan are obtained after 15-20 min. In children the radiation exposure to the gland is three- to fivefold higher than in adults.

For routine imaging a gamma camera with a pin-hole collimator is most often used. The patient lies in the supine position with the neck extended. Markers can be used to identify anatomic sites, such as the manubrium, or can be placed at the edge of a palpable nodule. Additional SPECT (single photon emission computed tomography), where the camera head rotates 180-360° around the patient, improves resolution and can provide volumetric estimates, but is not performed routinely [4, 5].

Thyroid uptake is influenced by the serum inorganic iodine level, which is dependent on the intake of iodine. A number of factors can influence the uptake. Thus, it is generally increased in hyperthyroid patients with Graves' disease or toxic nodular goiter, and decreased in patients with subacute or silent thyroiditis as well as in those with hypothyroidism (table 1) [6].

Indications for Thyroid Uptake and Imaging

When patients are referred for uptake and/or scan, it is important to ensure that they are not taking thyroid hormone. However, in congenital hypothyroidism, L-thyroxine therapy need not be delayed while awaiting scintigraphy, since scintigram validity depends on a normal or elevated TSH level, which is the case for many days after onset of treatment, during which time scintigraphy can be performed. It is also important to avoid the ingestion of excess iodine, and to secure that female adolescents are not pregnant.

Measurements of thyroid uptake and imaging give valuable information in several clinical situations (table 2). A known activity of tracer is administered orally, and the percentage accumulated at designated times is measured using either a probe or a gamma camera. It is almost standard procedure to obtain a 24-hour measurement, but the early 4- to 6-hour measurements allows the clinician to identify a thyroid with rapid turnover. Some obtain only an early measurement and by extrapolation calculate the 24-hour value [7]. The uptake is often used to determine therapy doses of 131I to treat patients with Graves' disease or toxic nodular goiter [8].

Only few studies, all retrospective, that describe the role of scintigraphy in the evaluation of the spectrum of pediatric thyroid disorders, have been published. In one study, comprising 280 children, indication for scintigraphy included hypothyroidism, neck masses, and hyperthyroidism [9] and was

Table 1. Factors that influence thyroid radioiodine uptake

Causes of increased uptake Hyperthyroidism Iodine deficiency

Rebound after withdrawal of antithyroid medication

Rebound after suppression of thyrotropin

Recovery phase of subacute, silent, or postpartum thyroiditis

Inborn errors of thyroid hormogenesis (apart from trapping defects)

Pregnancy (use of radiopharmaceuticals contraindicated during pregnancy)

Lithium carbonate therapy

Some patients with Hashimoto's thyroiditis

Causes of decreased uptake

Primary hypothyroidism

Destructive thyroiditis (subacute thyroiditis, silent thyroiditis, postpartum thyroiditis) Thyroidectomy, 131I treatment or external neck irradiation Thyroid hormone Antithyroid drugs

Excess iodine, including dietary supplements with iodine

Radiological contrast media

Amiodarone

Topical iodine

Perchlorate, thiocyanate

Sulphonamides, sulphonylurea

High-dose glucocorticosteroids

Table 2. Indications for thyroid uptake and imaging

Indications for measuring thyroid uptake Confirm the diagnosis of hyperthyroidism Differentiate different types of thyrotoxicosis Provide data for calculation of a therapeutic dose of 131I Detect intrathyroidal defects in organification Follow-up of patients treated for thyroid cancer Indications for thyroid scintigraphy Depict structure and function of the thyroid Differentiate different types of thyrotoxicosis Determine whether a nodule is functioning

Determine whether a cervical or mediastinal mass contains functioning thyroid Identify ectopic thyroid

Aid in the diagnosis of congenital hypothyroidism Identify thyroid metastases

Determine whether ablation therapy of thyroid cancer has been successful

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