This test is based on oral or intravenous administration of a radioactive iodine isotope to determine the iodine uptake of the thyroid tissue. Once in the body, the tracer is gradually taken up by the thy roid similar to the endogenous iodine pool. At a given time point, the amount of the tracer within the thyroid equilibrates with the rate of iodine uptake and thyroid hormone synthesis, and the re lease of the isotope into the circulation, incorporated in the thyroid hormones. Importantly, the test does not measure thyroid hormone synthesis and secretion, but merely the avidity of the thyroid gland for iodine, and its clearance rate relative to the renal iodine excretion. Thus, a clear inverse relationship exists between the dietary intake of iodine and the thyroid iodine uptake.

Procedure

An uptake measurement is usually performed 24 hours after oral administration of 3.7– 7.4 MBq of ¹²³I. The intrathyroidal amount of tracer can be measured with a gamma scintillation counter, placed at a fixed distance in front of the neck. The number of radioactive counts emitted from the thyroid is compared with the radioactivity administered to the patient. Mostly, the uptake value at 24 hours after radioiodine administration is used for practical reasons, and because most thyroid glands have reached the plateau of isotope accumulation at this time. A high 24 hour thyroid iodine uptake, usually in the range 40– 85%, is observed typically in patients with hyperthyroidism, but may be seen after recovery from thyroid failure or severe iodine deficiency leading to elevated thyroid- stimulating hormone (TSH) serum levels and hypothyroidism. In severe hyperthyroidism, seen in some patients with Graves’ disease, the thyroid uptake must be measured within 2– 6 hours after the radioiodine administration due to a very high iodine turnover rate. ^99mTc instead of iodine isotopes may be used in this situation since the thyroid iodine uptake in the very early period following the nuclide administration mainly reflects the NIS activity.

Indication for Thyroid Iodine Uptake Measurement

With accurate thyroid function markers, valid serological tests, and widely accessible ultrasound facilities, most thyroid patients can be diagnosed correctly. In patients with non- toxic benign goitre, an uptake measurement may indicate whether 131I therapy is a therapeutic option, since the 24- hour thyroid iodine uptake should be at least 20%. In hyperthyroid patients treated with 131I, the 24- hour uptake value— and sometimes the 131I half- life— is mandatory for calculation of the administered 131I activity at some centres.

Recombinant Human TSH Stimulation

Stimulation with 0.1 mg recombinant human TSH (rhTSH; doses of 0.005– 0.9 mg have been used)— given as a single intramuscular injection 24 hours before 131I administration— increases the 24- hour thyroid iodine uptake by 100% or more, even in iodine- loaded subjects. This effect correlates inversely with the baseline thyroid iodine uptake, with the implication that patients with the lowest baseline uptake potentially benefit the most from rhTSH stimulation in relation to 131I therapy (Figure 1). The effect on the 24- hour thyroid iodine uptake from rhTSH stimulation correlates inversely also with serum TSH.

Fig1. he positive effect of rhTSH stimulation on the thyroid 131I uptake (RAIU) in patients with multinodular non- toxic goitre is clearly inversely related to the initial RAIU. Reproduced with permission from Fast S, Nielsen VE, Grupe P, et al. Optimizing 131I Uptake After rhTSH Stimulation in Patients with Nontoxic Multinodular Goiter: Evidence from a Prospective, Randomized, Double- Blind Study. J Nucl Med 2009;50:732– 37. Copyright © 2009 the Society of Nuclear Medicine, Inc

Perchlorate Discharge Test

Some ions, like thiocyanate (SCN– ) and perchlorate (ClO4– ), inhibit the NIS, resulting in leakage from the thyroid, if the iodine is not intracellularly organified. Thus, the iodine loss from the thy roid gland following NIS inhibition correlates inversely with the thyrocyte’s ability to bind iodine.

The test is performed by administering a tracer dose of ¹²³I, and measurement of the thyroid ¹²³I uptake after 4 hours. Thereafter, 500– 1000 mg of NaClO4 (or KClO4) is administered orally, and the uptake measurement is repeated after 1 hour. Perchlorate acutely blocks the NIS, allowing non- organified ¹²³I to leak out of the thyrocyte. A thyroid organification defect is very likely to be present if the ¹²³I uptake after perchlorate administration shows a reduction of 15% or more. An ¹²³I uptake value in the range 10– 15% is considered borderline, and may be due to a partial organification defect. An abnormal perchlorate discharge test is seen in inborn defects of iodine organification, for example, human thyroid peroxidase (TPO) gene mutations or Pendred’s syndrome.

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مواضيع ذات صلة

Transport of Thyroid Hormone: Plasma Transport

Iodide Uptake

Biosynthesis of Thyroid Hormone

Regulation of Thyroid Function

Regulation of Thyroid Hormone Secretion : The Hypothalamic-Pituitary Thyroid Axis

Transport and Metabolism of Thyroid Hormones

−Secretion of T4 and T3 and Recycling of I

Thyroglobulin Storage, Endocytosis, and Breakdown

Thyroid Peroxidase and DUOX: Tyrosine Iodination and Coupling

Thyroglobulin

The Thyroid Epithelial Cell

Introduction to Thyroid Hormones