Investigations

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History and examination will rarely do more than raise the possibility of a thyroid cancer diagnosis. They will also occasionally point to the likely diagnosis of nonmalignant thyroid nodules. Investigations will be required to clinch the diagnosis. The algorithm in Figure 8.1 will assist in distinguishing alternate diagnoses. This algorithm assumes the GP has ready access to basic thyroid function tests, and the ability to refer to specialists if required for further evaluation. A diagram outlining specialist diagnosis and management procedures (Figure 1.8) is given in Chapter 1.

The sensitive thyroid-stimulating hormone assay (TSH) is an important initial test. It will distinguish between potentially overactive thyroid nodules and others.

Whether further tests over and above TSH should be performed depends on context. Further tests are inappropriate where: (1) a mul-tidisciplinary team exists, (2) other tests are not readily available to the GP, and (3) the patient can be seen by specialists in a reasonable time frame. Initiation of other investigations by the GP should be performed (1) in the absence of specialist services, (2) where the tests are available to the GP, and (3) where they can be performed ahead of the referral and taken to the specialist. This may actually save time.

Figure 8.1 General practitioner diagnosis of potential malignant thyroid nodule. Heavily shaded boxes = diagnostic tests, light shaded boxes = potential diagnosis.

If the TSH is suppressed, a pertechnetate (99mTc) or 123I scan should be ordered. An active nodule will demonstrate increased uptake of the radioactive material, which confirms the diagnosis of a hyperfunctioning isolated nodule, or dominant nodule in a hyperfunctioning gland. However, most nodules will not demonstrate increased uptake (so-called "cold" nodules). These will require fine-needle aspiration biopsy to determine their composition.

If the TSH is normal or elevated, the diagnosis then is between an autoimmune thyroiditis and other forms of nodule including cancer.

Thyroid autoantibody assays (antithyroid per-oxidase and antithyroglobulin) will determine the presence of immune activity. Although cancers and Hashimoto's thyroiditis can coexist, this is rare [7]. Serum calcitonin would be added to the investigations performed if there is a family history of medullary thyroid carcinoma.

Fine-needle aspiration biopsy is the only test that can confirm the presence of thyroid cancer. The test has a false-negative rate of 1-11%, a false-positive rate of 1-8%, a sensitivity of 65-98%, and a specificity of 72-100%. Limitations of fine-needle aspiration are related to the skill of the aspirator, the expertise of the cytol-ogist, and the difficulty in distinguishing some benign cellular adenomas from their malignant counterparts [12]. Ultrasound guided biopsy is useful in aspiration of impalpable lesions.

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