Thyroid Radioactive Iodine Uptake (RAIU)

Definition

A tracer dose of radioactive iodine (131I or 123I) is administered orally, and the radioactivity over the thyroid is measured at specific time intervals.
Normal range: 10 " “35% in 24 hours depending on local variations in iodine intake.

Evaluation of hyperthyroidism associated with low RAIU (e.g., factitious hyperthyroidism, subacute thyroiditis, struma ovarii)
Distinguish Graves disease from toxic nodular goiter
Assess function of nodules ( "hot " ¯ or "cold " ¯)
Determine location and size of functioning thyroid tissue
Detect metastases from differentiated thyroid cancers
Evaluate use of radioiodine therapy
Determine the presence of an organification defect in thyroid hormone production
In combination with T3 suppression test: Administration of triiodothyronine suppresses RAIU by >50% in the normal person but not in patients with Graves disease or toxic nodules; shows autonomy of TSH secretion. Infrequently used

Graves disease (diffuse toxic goiter)
Plummer disease (toxic multinodular goiter)
Toxic adenoma (uninodular goiter)
Thyroiditis (early Hashimoto; recovery stage of subacute thyroiditis)
TSH excess
- TSH administration
- TSH production by pituitary tumor (TSH >4 ˇ ¼U/mL) or other neoplasm
- Defective thyroid hormone synthesis
- Human chorionic gonadotropin " “mediated hyperthyroidism (e.g., choriocarcinoma, hydatidiform mole, embryonal carcinoma of the testis, hyperemesis gravidarum)

Hypothyroidism (tertiary, secondary, late primary)
Thyroiditis (late Hashimoto; active stage of subacute thyroiditis; RAIU does not usually respond to TSH administration)
Thyroid hormone administration (T3 or T4)
- Therapeutic
- Factitious (RAIU is augmented after TSH administration)
Antithyroid medication
Iodine-induced hyperthyroidism (Jodbasedow)
X-ray contrast media, iodine-containing drugs, iodized salt
Graves disease with iodine excess
Ectopic hypersecreting thyroid tissue
Metastatic functioning thyroid carcinoma
Struma ovarii
Drugs (e.g., calcitonin, thyroglobulin, corticosteroids, dopamine)

Contraindications: pregnancy, lactation, childhood.
Not valid for 2 " “4 weeks after administration of antithyroid drugs, thyroid, or iodides; the effect of organic iodine (e.g., x-ray contrast media) may persist for a much longer period.
Because of widespread dietary use of iodine in the United States, RAIU should not be used to evaluate euthyroid state.
Increased by withdrawal rebound (thyroid hormones, propylthiouracil), increased iodine excretion (e.g., diuretics, nephrotic syndrome, chronic diarrhea), decreased iodine intake (salt restriction, iodine deficiency).