The Thyroid Scintigram

The thyroid scintigram is a map of the distribution of radioactive iodine within the thyroid gland. To extract clinically useful information from it, correlation with palpable thyroid abnormalities, pathophysiology, and microscopic anatomy is required. The initial attempt at such correlation was undertaken in cases presenting a problem in preoperative identification of the malignant nodule. As the low level of functional activity of thyroid cancer was well known, it seemed reasonable to attempt to correlate apparent function in terms of scintigram activity and pathological anatomy (1–7). The first classification compared the radioactivity in suspect areas of the scintigram with that in areas presumed to represent the normal portions of the thyroid gland. The basis of this classification was a visual inspection of the concentration of marks recorded or the degree of blackening of a photographic film. The darker a given area on the scintigram the greater the radioactivity (and presumably the function) of the corresponding area of the thyroid as seen by the detector. The descriptive terms “hot nodule,” “warm nodule,” “cool nodule,” and “cold nodule” were introduced into the medical literature to express the above comparisons (8–11). These inexact estimates gained favor, since technical considerations in the production of the scintigram made absolute standards of comparison impractical. Definition A “hot” nodule has usually been defined as one concentrating radioactive iodine to a greater degree than the remaining thyroid tissue (1, 12–14). This diagnosis has carried with it the implication of localized hyperfunction (1, 15). In an attempt to define what should be accepted as a “hot nodule” in a more quantitative fashion, Perlmutter and Slater suggested that the count rate should be 125 per cent or more of the symmetrical contralateral portion of the gland (11). The emphasis on the physical appearance of the scan and relative count rates has delayed recognition of the fact that the scintigram pattern described above may be associated with diverse pathophysiology. All the following conditions would qualify as “hot nodules” as currently defined: (a) The autonomous hyperfunctioning thyroid lesion. (b) Tissue of relatively normal function with surrounding areas of degeneration or thyroiditis. (c) A normal, hyperplastic, or nodular lobe with congenital or acquired absence of the other lobe. “Acquired” includes total or subtotal surgical resection. (d) A prominently lobulated gland with sufficient asymmetry that by virtue of greater mass alone one lobe or one area contains significantly more radioactive iodine than the other. (Toxic dependent goiter Graves' disease may present in such a fashion.) Perlmutter and Slater even made reference to a functioning cervical node metastasis following total thyroidectomy for thyroid carcinoma as an example of a “hot nodule.”