The test does not measure hormone production and release but merely the avidity of the thyroid gland for iodide and its rate of clearance relative to the kidney. Disease states resulting in excessive production and release of thyroid hormone are most often associated with increased thyroidal RAIU and those causing hormone underproduction with decreased thyroidal RAIU (Figure 6-2, below). Important exceptions include high uptake values in some hypothyroid patients and low values in some hyperthyroid patients. Increased thyroidal RAIU with hormonal insufficiency co-occur in the presence of severe iodide deficiency and in the majority of inborn errors of hormonogenesis (see Chapter 20 and 16 ). In the former, lack of substrate, and in the latter, a specific enzymatic block of hormone synthesis cause hypothyroidism poorly compensated by TSH-induced thyroid gland overactivity. Decreased thyroidal RAIU with hormonal excess is typically encountered in the syndrome of transient thyrotoxicosis (both de Quervain’s and painless thyroiditis), 9 ingestion of exogenous hormone (thyrotoxicosis factitia), iodide-induced thyrotoxicosis (Jod-Basedow disease), 10 and in patients with thyrotoxicosis on moderately high intake of iodide (see Table 6-3 ). High or low thyroidal RAIU as a result of low or high dietary iodine intake, respectively, may not be associated with significant changes in thyroid hormone secretion.