The role of nuclear medicine in the clinical management of benign thyroid disorders. Part 1. Hyperthyroidism

Abstract

Benign thyroid disorders, especially hyper and hypothyroidism are the most prevalent endocrine disorders. The most common etiologies of hyperthyroidism are autoimmune hyperthyroidism (Graves’ disease, GD), toxic multinodular goiter (TMNG) and toxic thyroid adenoma (TA). Less common etiologies include destructive thyroiditis (e.g., amiodarone-induced thyroid dysfunction), and factitious hyperthyroidism. GD is caused by autoantibodies against the TSH receptor. TMNG and TA are caused by a somatic activating-gain-of-function mutation. Typical laboratory findings in patients with hyperthyroidism are low TSH, elevated free-T4 and free-T3) as well as TSH-receptor autoantibodies in patients with GD. Ultrasound imaging is used to determine the size and vascularity of the thyroid gland, and location, size, number and characteristics of thyroid nodules. Combined with lab tests, these features constitute the first-line diagnostic approach to distinguish different forms of hyperthyroidism. Thyroid scintigraphy with either radioiodine or ^99mTc-Pertechnetate scintigraphy is useful to characterize different forms of hyperthyroidism and provides information for planning radioiodine therapy. There are specific scintigraphic patterns for GD, TMNG, TA, and for destructive thyroiditis. Scintigraphy with ^99mTc-Sestamibi allows differentiation of type-1 and type-2 amiodarone-induced hyperthyroidism. The radioiodine uptake test provides information for planning radioiodine therapy of hyperthyroidism. Hyperthyroidism can be treated with oral anti-thyroid drugs, surgical thyroidectomy, or ¹³¹I-iodide. Radioiodine therapy is generally considered after failure of treatment with anti-thyroid drugs, and/or when surgery is contraindicated or refused by the patient. In patients with TA or TMNG the goal of radioiodine therapy is to achieve euthyroid status. In GD, the goal of radioiodine therapy is to induce hypothyroidism, a status that is readily treatable with oral thyroid hormone replacement therapy. Dosimetric estimates based on the thyroid volume to be treated and on radioiodine uptake should guide selection of the ¹³¹I-activity to be administered. Early side-effects of radioiodine therapy (typically mild pain in the thyroid) can be handled by non-steroidal anti-inflammatories. Delayed side-effects following radioiodine therapy for hyperthyroidism are hypothyroidism and the minimal risk of radiation-induced malignancies.