Thyroid, radioactive iodine scan

Ladenson PW, Braverman LE, Mazzaferri EL, et al. Comparison of administration of recombinant human thyrotropin with withdrawal of thyroid hormone for radioactive iodine scanning in patients with thyroid carcinoma. N Engl J Med 1997 337 888-896. 

Different forms of radioiodine have been used at different times, including 123I, 12SI, and 131I. Radioactive iodine is used to scan the thyroid gland and in the treatment of thyrotoxicosis. See in the monograph on Radioactive iodine. 

A scan of the radiation released by radioactive iodine concentrated in thyroid tissue gives an image of the thyroid gland. 

Radioactive iodine finds its widest use in the treatment of hyperthyroidism and in the diagnosis of disorders of thyroid function. Sodium iodide (lodotope Therapeutic) is available as a solution or in capsules containing essentially carrier-free suitable for oral administration. Sodium iodide is available for scanning procedures. Discussion here is limited to the uses of... 

A considerable advance in the management of thyroid cancer has resulted from the introduction of recombinant human TSH (thyrogen). A dose of 0.9 mg is administered intramuscularly, followed by an identical dose 24 hours later. The serum thyroglobuLin is then measured 48-72 hours after the second TSH injection this has become the preferred diagnostic test for following patients with differentiated thyroid cancer who have had thyroidectomy and ablation of any remnant tissue with radioactive iodine. This regimen can also be used to stimulate iodine uptake by malignant cells in a total body scan (see below). 

After consumption of Na l, the patient s thyroid is scanned for radioactivity levels to determine the efficiency of iodine absorption, (a) Scan of radioactive iodine in a normal thyroid (b) Scan of an enlarged thyroid... 

Surgical resection of the thyroid (total or near total thyroidectomy) followed by remnant ablation with radioactive iodine (RAI) therapy to destroy the remaining normal or cancerous thyroid tissue is considered the standard of care for all but small (less than 1—1.5 cm) thyroid cancers, and is associated with decreased frequency of local recurrences and an improvement in overall survival rate (Mazzafferi and Jhiang, 1994 Mazzafferi and Kloos, 2001). Furthermore, the elimination of uptake by residual normal thyroid tissue enhances the sensitivity of subsequent scanning and serum thyroglobufln (Tg) for detection of recurrent or persistent disease. 

When total or near-total thyroidectomy is the intention of the surgical team, patients are typically referred for a postoperative scan for assessment for administration of radioactive iodine for removal of remnant thyroid tissue. Total resection of the thyroid is a difficult proposition, and attention to critical structures within the neck usually interferes with complete surgical removal of gland tissue. Radioiodine ablation refers to the use of 1311 to destroy what is presumed to be normal thyroid tissue remaining within the... 

Radioisotope scanning was used in the early days to detect filling defects in organs, such as the liver and spleen. Radioactive sodium iodide was used to image the thyroid. Radioactive particles were used to image the reticuloendothelial system. The dye, radio-iodinated Rose Bengal, was used to image the liver. 

The most important prognostic factor for all types of thyroid carcinoma is the TNM status including the invasion of blood and lymph vessels and metastatic spread. For the monitoring of thyroid carcinoma two methods are usually used the nuclear body-scan with radioactive iodine and measuring of serum thyroglobulin, but we have to consider that both methods can reliably detect systemic metastatic spread or relapse of thyroid carcinoma only, if thyroid tissue has been completely removed. 

Figure 10.6 A thyroid scan produced after the administration of a radioactive iodine isotope. Is either a hot spot or a cold spot present ...

FIGURE 16.5 (a) A scanner is used to detect radiation from a radioisotope that has accumuiated in an organ. (b)A scan of the thyroid shows the accumuiation of radioactive iodine-131 in the thyroid. 

Since iodine concentrates in the thyroid gland, a scan of the thyroid is conducted using a radioactive isotope of iodine, 1-131, which has a half-life of 8.0 days. The iodine sample can be administered by swallowing a capsule or Uquid. Only a small amount of radioactive iodine is used for the procedure, and after a week the level of radioactivity is negligible. Iodine-131 can also be administered to treat goiters. 

Uptake of radioactive iodine isotopes can be used to scan the gland, and determine the affinity of the gland to introduced iodine, and is a measure of deficiency. The most common isotope used is because of its relatively short 13-h half-life and 7 photon emission. Uptake is increased in iodine deficiency. Isotopes can also be used to examine the organification of iodine in the formation of thyroid hormones. This is an impractical method for surveying populations. Table 3 provides the WHO criteria for defining iodine deficiency as a public health problem. 

Although iodine has several radioactive isotopes, greatest use has been made of I. It has a half-life of 8 days therefore, more than 99% of its radiation is expended within 56 days. Its radioactive emissions include both y rays P and particles. The short-lived radionuclide of iodine, I, is primarily a y-emitter with a half-life of only 13 hours. This permits a relatively brief exposure to radiation during thyroid scans. 

After consumption of Na l, the patient s thyroid is scanned for radioactivity levels to determine the efficiency of iodine absorption. 

During the early recovery phase of the disease, the radioactive uptake of the thyroid was measurable, and thyroid scans show a homogenous distribution of the tracer in all the patients. The iodine content of the gland increased only in 3 patients (from +35 to + 105 % over the initial values respectively) (Table 2). However in one of these 3 patients, the iodine content decreased again from 13.5 to 6 mg 17.5 months after the end of therapy. In the other patients, iodine contents decreased (from - 32 % to - 100 % of the initial value) even when the disease was developed on a previous goitrous gland. Nearly complete iodine depletion was observed in 4 patients. No correlation was found between the decreases in the thyroid... 

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