Anatomic and Tissue Diagnoses
The purpose of the procedures described in this section is to evaluate the anatomic features of the thyroid gland, localize and determine the nature of abnormal areas and eventually provide a pathologic or tissue diagnosis. All of these tests are performed in vivo.
Normal and abnormal thyroid tissue can be externally imaged by three scintiscanning methods: (1) with radionuclides that are concentrated by normal thyroid tissues such as iodide isotopes, and 99mTc given as the pertechnetate ion; (2) by administration of radiopharmaceutical agents which are preferentially concentrated by abnormal thyroid tissues; and (3) fluorescent scanning, which uses an external source of 241Am and does not require administration of radioactive material. Each has specific indications, advantages, and disadvantages.
The physical properties, dosages, and radiation delivered by the most commonly used radioisotopes are listed in Table 6-2. The choice of scanning agents depends on the purpose of the scan, the age of the patient, and the equipment available. Radioiodide scans cannot be performed in patients who have recently ingested iodine-containing compounds. 123I and 99mTcO4- are the radionuclides of choice because of the low radiation exposure.355-357 Iodine-131 is still used for the detection of functioning metastatic thyroid carcinoma by total body scanning.
Radioiodide and 99mPertechnetate Scans. 99mTcO4- is concentrated, and all iodide isotopes are concentrated and bound, by thyroid tissue. Depending upon the isotope used, scans are carried out at different times after administration: 20 minutes for 99mTcO4-, 4 or 24 hours for 123I-; 24 hours for 125I- and 131I-; and 48, 72, and 96 hours when 131I- is used in the search for metastatic thyroid carcinoma. The appearance of the normal thyroid gland on scan may be best described as a narrow-winged butterfly. Each "wing" represents a thyroid lobe, which in the adult measures 5 ? 1 cm in length and 2.3 ? 0.5 cm in width.358 Common variants include the absence of a connecting isthmus, a large isthmus, asymmetry between the two lobes, and trailing activity extending to the cricoid cartilage (pyramidal lobe). The latter is more commonly found in conditions associated with diffuse thyroid hyperplasia. Occasionally, collection of saliva in the esophagus during 99mTcO4- scanning may simulate a pyramidal lobe, but this artifact can be eliminated by drinking water.
The indications for scanning are listed
in Table 6-8. In clinical practice, scans
are most often requested for evaluation of the functional activity of solitary
nodules. Normally, the isotope is homogeneously distributed throughout both
lobes of the thyroid gland. This distribution occurs in the enlarged gland of
Graves' disease and may be seen in Hashimoto's thyroiditis. A mottled appearance
may be noted in Hashimoto's thyroiditis and can occasionally be seen in Graves'
disease especially after therapy with radioactive iodide. Irregular areas of
relatively diminished and occasionally increased uptake are characteristic of
large multinodular goiters. The traditional nuclear medicine jargon classifies
nodules as "hot", "warm," and "cold," according
to their isotope-concentrating ability relative to the surrounding normal parenchyma
(Figure 6-6). Hot, or hyperfunctioning,
nodules are typically benign, although the presence of malignancy has been reported.359,360
Cold, or hypofunctioning, nodules may be solid or cystic. Some may prove to
be malignant, but the great majority are benign. This differentiation cannot
be made by scanning.77,361
Occasionally, a nodule which is functional on a 99mTcO4- scan will be found
to be cold on an iodine scan; this pattern is found with both benign and malignant
nodules. The scan is of particular value in identifying autonomous thyroid nodules
since the remainder of the gland is suppressed. Search for functioning thyroid
metastases is best accomplished using 2-10 mCi of 131I after ablation of the
normal thyroid tissue and cessation of hormone therapy to allow TSH to increase
above the upper limit of normal. Recent studies have addressed the question
of whether recombinant human TSH allows scanning without requiring cessation
of hormone therapy.362
Uptake is also found outside the thyroid gland in patients with lingual thyroids
and in the rare ovarian dermoid tumor containing functioning thyroid tissue.
 |
| Figure 6-6. Thyroid Scans. (a) Normal thyroid imaged with 123I. (b) Cold nodule in the right lobe imaged by 99mTc. (c) Elderly woman with obvious multinodular goiter and the corresponding radioiodide scan on the right. |
|
Table 6-8. Indications for Radionuclide Scanning |
|
Detection of anatomic variants and search for
ectopic thyroid tissue |
The scan can be used as an adjunct during TSH stimulation and T3 suppression tests to localize suppressed normal thyroid tissue or autonomously functioning areas, respectively (see below). Applications other than those listed in Table 6-8 are of doubtful benefit and are rarely justified considering the radiation exposure, expense, and inconvenience. 123I single photon emission computed tomography (SPECT) may also be useful in the evaluation of thyroid abnormalities.363
Other Isotope Scans. Because most test procedures, short of direct microscopic examination of thyroid tissue, fail to detect thyroid malignancy with any degree of certainty, efforts have been made to find other radioactive materials that would hopefully be of diagnostic use. Several such agents that are concentrated by metabolically active tissues, irrespective of whether they have iodide-concentrating ability, have been tried. However, despite claims to the contrary, they have either had only limited value or their diagnostic usefulness has not been fully evaluated. These agents include 75Se methionine, 125Ce, 67Ga, citrate, 32P, pyrophosphate 99mTc, and 201Thallium.364
Scanning with 131I-labeled anti-TG for the detection of occult metastatic thyroid malignancy that fails to concentrate 131I showed early promising results.365 However, the procedure has not proved clinically useful.
Ultrasonography, or echography, is used to outline the thyroid gland and to characterize lesions differing in density from the surrounding tissue. The technique differentiates interphases of different acoustic densities, using sound frequencies in the megahertz range that are above the audible range. A transducer fitted with a piezoelectric crystal produces and transmits the signal and receives echo reflections. Interfaces of different acoustic densities reflect dense echoes, liquid transmits sound without reflections, and air-filled spaces do not transmit the ultrasound.368
One of the most useful applications of the ultrasonogram is the differentiation of solid from cystic lesions.368,369 Purely cystic lesions are entirely sonolucent, whereas solid lesions produce multiple echoes due to multiple sonic interphases. Many lesions, however, are mixed (solid and cystic) called complex lesions. Some tumors may have the same acoustic characteristics as the surrounding normal tissue thus, escaping echographic detection. While high-resolution ultrasonography can detect thyroid nodules of the order of few millimeters,370 lesions need to be larger than 0.5 cm to allow differentiation between solid and cystic structures. A sonolucent pattern is frequently noted in glands with Hashimoto's thyroiditis, but this has also been described in multinodular glands and in patients with Graves' disease.368,371,372
Because sonography localizes the position as well as the depth of lesions, the procedure has been used to guide the needle during aspiration biopsy.373 In complex lesions, the sonographic guiding insures sampling from the solid portion of the nodule. With experience and proper calibration, sonography can be used for the estimation of thyroid gland size.374,375 Several recent reports have described treatment of toxic nodules by the injection of alcohol under sonographic guidance.376 Although ultrasonography has found virtually the same applications as scintiscanning, claims that the former may differentiate benign from malignant lesions are unfounded. Also, ultrasonography cannot be used for the assessment of substernal goiters because of interference from overlying bone.
The procedure is simple and painless, and at the frequencies of sound used, do not produce tissue damage. Since it does not require the administration of isotopes, it can be safely used in children and during pregnancy. Also, because the procedure is independent of iodine-concentrating mechanisms, it is valuable in the study of suppressed glands.
A simple X-ray film of the neck and upper mediastinum may provide valuable information regarding the location, size, and effect of goiter on surrounding structures. X-rays may show an asymmetric goiter, an intrathoracic extension of the gland, and displacement or narrowing of the trachea. If there is any suggestion of posterior extension of the mass, it is useful to take films during the swallow of X-ray contrast material. The soft tissue X-ray technique may disclose calcium deposits. Large deposits in flakes or rings are typical of an old multinodular goiter, whereas foci of finely stippled flecks of calcium are suggestive of papillary adenocarcinoma.
Information, not related to anatomic abnormalities of the thyroid gland may be obtained from X-ray studies. In children with a history of hypothyroidism, an X-ray film of the hand to determine the bone age could aid in estimating the onset and duration of thyroid dysfunction.294,295 Hypothyroidism leads to retardation in bone age and in infants produces a dense calcification of epiphyseal plates most easily seen at the distal end of the radius. Long-standing myxedema produces pituitary hypertrophy which, especially in children but also in adults, causes enlargement of the sella turcica demonstrable on imaging of the pituitary region.
Computed Tomography (CT) and Magnetic Resonance Imaging (MRI). These techniques provide useful information on the location and architecture of the thyroid gland as well as its relationship to surrounding tissues.378 They are, however, too costly relative to other procedures which provide similar information. An important application of CT is the assessment and delineation of obscure mediastinal masses and large substernal goiters.379 The necessity to infuse iodine containing contrast agents limits the application of CT in patients being considered for radioiodide therapy. CT and MRI have found firm application in another area of thyroid diseases, namely, in the evaluation of ophthalmopathy343 and mediastinal masses.379
A barium swallow may be useful in evaluating impingement of a goiter on the esophagus, while a flow volume loop 380 may be useful in documenting functional impingement on the upper airway.
