RADIOIODINE UPTAKE AND THYROID SCINTISCANNING

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Decades ago, the measurement of thyroidal radioactive iodine uptake and thyroid scintigraphy were routinely used to assess thyroid function. This application has largely been replaced by sensitive in vitro tests of thyroid function. Radioactive iodine uptake remains an important technique in evaluating the hyperthyroid patient, and thyroid imaging is valuable in the work-up of hyperthyroidism, thyroid nodules, differentiated thyroid cancer, and ectopic thyroid tissue. This article reviews current uses of radioactive iodine uptake testing and radionuclide thyroid scanning in thyroid conditions other than established thyroid cancer. Other thyroid imaging modalities and the role of nuclear medicine in thyroid cancer are discussed elsewhere.

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EMBRYOLOGY AND ANATOMY

A review of thyroid embryology is necessary to understand the many variations in location and configuration of benign functioning thyroid tissue that are encountered in thyroid scintiscanning. The thyroid gland develops from a thickening in the midline of the anterior pharyngeal floor, first noticeable about the sixteenth day of gestation. It expands as it descends ventrally while remaining attached to the pharyngeal floor by a stalk called the thyroglossal duct. This stalk begins to atrophy at

RADIONUCLIDES USED FOR THYROID SCANNING AND RADIOACTIVE IODINE UPTAKE

Only three radionuclides are used in routine clinical practice (Table 1). Of the two iodine radioisotopes, iodine-123 (123I) delivers far less radiation because of its short half-life (13 hours) and the absence of beta radiation. Its gamma photon energy of 159 keV is ideally suited for thyroid scanning. Previously, commercially available 123I had longer lived 124I and 125I contaminants. Reactor-produced123I is much purer, with virtually no 124I and less than 1%125I. Currently, 123I is available

MEASUREMENT OF RADIOIODINE UPTAKE

Radioactive iodine uptake was the first diagnostic nuclear medicine procedure. It was performed in 1940 in a primitive way using a Geiger counter. Contemporary methodology was introduced by Werner34 about a decade later. As in vitro thyroid function studies gradually became more sensitive, radioactive iodine uptake measurement was used less frequently as a measure of thyroid function. Its main role is in the evaluation of the hyperthyroid patient to distinguish subacute or silent thyroiditis

CLINICAL INTERPRETATION OF RADIOACTIVE IODINE UPTAKE

A variety of thyroid states are associated with abnormal radioactive iodine uptake measurements, and other factors can interfere with the value (Table 2). Causes of abnormal results include the following:

  • Increased uptake

    • Hyperthyroidism

    • Hashimoto's thyroiditis

    • Iodine deficiency

    • Subacute, silent, or postpartum thyroiditis in the recovery phase

    • Rebound after withdrawal of antithyroid drugs

    • Enzymatic defects in thyroid hormone biosynthesis

    • Choriocarcinoma and hydatidiform mole

    • Lithium

THYROID SCINTIGRAPHY

Thyroid scintigraphy or scintiscanning (hereafter referred to as scanning) permits visualization of the thyroid gland and functioning thyroid tissue elsewhere in the body. With the widespread use of fine-needle aspiration biopsy (FNAB) and ultrasound imaging, the role of thyroid scanning in initial evaluation of thyroid nodules has diminished. Nevertheless, there are clear indications for obtaining thyroid scans. The important role of 131I scanning in the postoperative evaluation of the patient

CLINICAL UTILITY OF THYROID SCANNING

Many thyroid conditions have been studied with scans. Current clinical uses of thyroid scanning include the following:

  • 1

    Demonstrating that a palpable enlargement represents an entire lobe rather than a nodule—hemiagenesis, asymmetric normal thyroid, firm Hashimoto's disease

  • 2

    Localizing functioning thyroid tissue—ectopic thyroid tissue, nonthyroid neck masses

  • 3

    Identifying the cause of congenital hypothyroidism

  • 4

    Identifying the cause of hyperthyroidism

  • 5

    Identifying functioning thyroid nodules

  • 6

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