Thyroid Radioactive Iodine Uptake (RAIU)

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Subject: Thyroid Radioactive Iodine Uptake (RAIU)

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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.

Use

  • 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

Interpretation

Increased In

  • 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 μU/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)

Decreased In

  • 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)

Limitations

  • 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).

 
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