Radionuclide imaging is often used in the diagnosis and work-up of a wide range of neoplasms, on the basis of the biologic behavior of the tumor. Neuroendocrine tumors are a subgroup of neoplasms that are generally small and slow growing, and consequently their identification with conventional anatomic imaging can be difficult. Depending on the physiologic properties of the tumor, functional images obtained with radionuclides are often complementary to anatomic images, not only in the localization of the tumor and its metastases, but also in the assessment of prognosis and response to therapy. Familiarity with the choice of the appropriate radiopharmaceutical, proper imaging protocols, and the wide range of imaging patterns will enable the radiologist to guide the clinician in case management.

The term thyrotoxicosis refers to the clinical syndrome of increased systemic metabolism that results when the serum concentrations of free thyroxine, free triiodothyronine, or both are elevated. The term hyperthyroidism refers to overactivity of the thyroid gland with a resultant increase in thyroid hormone synthesis and release into the systemic circulation. These terms are not interchangeable, since thyrotoxicosis can develop in thyroid conditions that are not associated with increased thyroid function, such as thyroiditis, or in so-called factitious hyperthyroidism. The clinical signs and symptoms of thyrotoxicosis are virtually identical regardless of the cause. However, in a given patient, every attempt should be made to determine the exact cause of the thyrotoxicosis, as this in turn determines the prognosis and treatment. Since thyroid scintigraphy demonstrates the functional state of the thyroid gland, it should be used, in conjunction with determination of radioactive iodine uptake, as the imaging modality of choice for diagnosis of thyrotoxicosis. Although the scintigraphic features of several of the thyroid disorders that cause thyrotoxicosis may overlap, their recognition helps narrow the differential diagnosis, thereby guiding the referring physician in the work-up and management of this disorder.

The term autoimmune thyroiditis encompasses multiple inflammatory conditions of the thyroid gland, each with variable clinical manifestations. The more acute forms, silent (painless) thyroiditis and postpartum thyroiditis, are associated with transient hyperthyroidism and are sometimes mistaken for Graves disease. The chronic form, Hashimoto thyroiditis (chronic autoimmune thyroiditis), results in goiter and eventual hypothyroidism unless it is recognized and treated promptly. Thyroid uptake measurements and scintigraphic findings (usually obtained with technetium-99m or iodine-123) play a complementary role along with thyroid function testing in differentiating autoimmune thyroiditis from other thyroid diseases, thereby influencing treatment. In some cases, histologic evaluation of biopsy specimens is required to yield the definitive diagnosis. Knowledge of the entire spectrum of these disorders is essential for appropriate case management.

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Subclinical hyperthyroidism is defined as normal serum free thyroxine and a free triiodothyronine level, with a thyroid-stimulating hormone level suppressed below the normal range and is usually undetectable. Although patients with this diagnosis have no or few signs and symptoms of overt thyrotoxicosis, there is sufficient evidence that it is associated with a relatively higher risk of supraventricular arrhythmias as well as the acceleration or the development of osteoporosis. Consequently, the approach to the patient with subclinical hyperthyroidism is controversial, that is, therapeutic intervention versus watchful waiting. Regardless, it is imperative for the referring physician to identify the causative thyroid disorder. This is optimally accomplished by a functional study, namely scintigraphy. Recognition of the scan findings of the various causes of subclinical hyperthyroidism enables the imaging specialist to help in diagnosing the underlying condition causing thyroid-stimulating hormone suppression thereby facilitating the workup and management of this thyroid disorder.