Thyroid uptake

Thyroid Uptake Systems. Studies involving absolute thyroid uptake can be performed without imaging using small amounts of or and a simple scintillation probe. This is caUbrated using a phantom, ie, a model of a portion of the human body, loaded with the isotope being used. This instmment is also useful for assaying thyroid exposure to radioiodine among personnel. 

Subsequent to the ingestion of iodine in various forms, I is absorbed by the small intestine and enters the blood. Two competing pathways are involved in the clearance of I from the blood renal filtration into urine and thyroidal uptake. The renal clearance rate for I (30-50 mL/minute) varies only with the glomerular filtration rate. However, the thyroidal 1 clearance rate is autoregulated to maintain an absolute thyroidal I uptake rate of approximately 100 jig I each day. To accomphsh this, the thyroidal I clearance rate may vary (3 to 100 mL/minute) depending on the concentration of I in the blood. 

The ability of KI to block the thyroidal uptake of I and its incorporation into Tg would prove useful in the event of an accident at a nuclear power plant. In such an event, large quantities of radionuclides, including isotopes of radioiodine, could be released into the atmosphere. Administration of KI Thyro-Block) to inhibit the uptake and incorporation of radioiodine would be the most effective means of limiting the potential damage to the thyroid gland. 

T3 suppression test PO 75-100 meg/day for 7 days then repeat thyroid uptake test. 

Experimental design Groups of 8-11 male rats were treated with 0, 1, 3, or 6 mg/kg/day doses of an unspecified mixture of PBBs in lecithin liposomes by gavage for 10 days. Plasma was assayed on treatment days 10 and 20. Other end points were evaluated on treatment day 20 these included plasma TSH levels, 5-hour thyroid uptake of I, incorporation of into monoiodotyrosine, diiodotyrosine, I, or T4, amount of intrathyroidal iodide, thyroid and liver weights, and body weights. Differences between mean values for the measured parameters in the control and PBB-treated groups were analyzed with the Student s Mest, with aP value of 0.05 considered as statistically significant. 

The inhalation of trace amounts of [ 32i]niethyl iodide was followed by a decrease in plasma radioactivity, a thyroid uptake pattern and urinary excretion that were similar to those observed after oral administration of inorganic iodide (LARC, 1986). 

The use of iodine has been held responsible for the increasing frequency of relapse of Graves disease in the USA. Treatment of more severe cases of iodine-induced hyperthyroidism can be difficult, as thyroid synthesis inhibitors are not immediately active and 131I cannot be used because of low thyroid uptake. The carefully supervised combination of perchlorate and methimazole is effective (40), but surgery has also occasionally been advocated. 

A 72-year-old woman with dilated cardiomyopathy was given amiodarone for fast atrial flutter and 6 months later developed abnormal thyroid function tests, with a suppressed TSH and a raised serum thyroxine. The autoantibody profile was negative and a thyroid uptake scan showed reduced uptake (44). 

Castenfors H, Allgoth AM. The effect of iodochlorohy-droxyquinoline, diiodohydroxyquinoline and dichlorohy-droxyquinaldine on the thyroid uptake of radio-iodine. Scand J Clin Lab Invest 1957 9(3) 270-2. 

In order to minimize thyroid uptake of free radioactive iodine derived from radiolabelled allergen deiodination, saturated potassium iodide solution was given to the subjects in the 5 days before the study followed by potassium perchlorate for 2 days, starting from the day of the study. 

Thyroid clearance of exogenously administered monoclonal antibodies is reported to be affected by the concentration and injection volume of the administered drug. Following intraperitoneal administration of irrelevant CDR-containing mouse IgG2a monoclonal antibody to rats, thyroid uptake of radiolabeled mouse IgG2a monoclonal antibody has been demonstrated for higher doses and more concentrated solutions while lower doses or less concentrated solutions remain within blood vessels and do not penetrate the thyroid parenchyma [192],... 

This method may be used in the context of repeated dose treatment, on satellite groups of rats which show changes in thyroid uptake of labelled iodine. It may be sufficient to determine radio-iodine uptake in controls and in the high-dose group. 

Topical application of iodine-containing antiseptics to neonates has caused hypothyroidism. Iodide intake above that in a normal diet will depress thyroid uptake of administered radioiodine, because the two forms will compete. 

The adverse effects of radioiodine are as for iodism, above. In the event of inadvertent overdose, large doses of sodium or potassimn iodide should be given to compete with the radioiodine for thyroid uptake and to hasten excretion by increasing iodide turnover (increased fluid intake and a diuretic are adjuvants). 

Martino E, Bartalena L, Mariotti S, Aghini-Lombardi F, Ceccarelli C, Lippi F, Piga M, Loviselli A, Braverman L, Safran M, et al. Radioactive iodine thyroid uptake in patients with amiodarone-iodine-induced thyroid dysfunction. Acta Endocrinol (Copenh) 1988 119(2) 167-73. 

Iodides are excreted in high concentrations in breast milk. The use of the short half-life I in preference to I partly overcomes this problem, but iodide or perchlorate, which is used to block thyroid uptake in the mother can be excreted in the milk. Likewise, furosemide or chole-cystokinin, which can be used to alter the distribution of radiopharmaceuticals, can be excreted in breast milk. 

Although potassium iodide does not protect the thyroid from external radiation, patients suffering from internal radioiodine contamination should receive potassium iodide to prevent or reduce thyroid uptake. To be effective, patients must receive the potassium iodide within a few hours after exposure (5,11). Compared to adults, children are more susceptible to the effects of radioiodine. Consequently, the Federal Drug Administration (15) and World Health Organization recommendations for administration of potassium iodide differ for children and adults. Table 4.6 contains the FDA recommendations for potassium iodide administration. 

The biodistribution pattern of I-DOTATATE (Fig. 8.15) shows no uptake in major organs of interest and rapid clearance of the activity. Low thyroid uptake was indicative of the in vivo stability of the product. 

The biological uptake pattern of the radioiodinated peptide indicates urinary as well as gastrointestinal excretion. In vivo metabolization is evidenced by the thyroid uptake (see Table 16.2), which reached 10.2 8.5% of the injected dose at 24 h. In the case of the Lu labelled peptide, the elimination is mainly through urinary excretion. Dose estimates for the radioiodinated peptide are shown in Table 16.4. 

TSH) in 60% of patients, despite attempts at blocking thyroid uptake of free using Lugols iodine and saturated solution of Kl. 

Several drugs interfere with the normal biodistribution of Tc-pertechnetate (Hla-dik et al. 1987). Thus, cancer chemotherapeutic agents (methotrexate) can affect brain scintigraphy atropine, isoprenaline, and analgesics interfere in abdominal imaging iodine and other blockers (perchlorate, perrhenate) can modify thyroid uptake. 

In certain clinical situations, when thyroidal uptake of Tc-pertechnetate should he avoided, pretreatment with an oral dose of potassium perchlorate is used to inhibit uptake. Perchlorate anion shows greater affinity for the transporter than does iodide and is, therefore, a competitive inhibitor of the thyroid iodide trap (Wolff and Maurey 1962). Like pertechnetate, perchlorate is concentrated in the thyroid and is not metabolized. Assuming 100% resorption of an oral dose of 300 mg of KCIO4 and an initial distribution volume of 20 1, the resulting concentration of perchlorate in blood is approximately 10M, sufficient for saturation of the thyroid trapping mechanism (Loberg 1979). 

Long-term administration of salicylates decreases thyroidal uptake and clearance of iodine but increases O2 consumption and the rate of disappearance of thyroxine and triiodothyronine from the circulation. These effects probably are caused by the competitive displacement by salicylate of thyroxine and triiodothyronine from binding proteins in plasma (see Chapter 56). 

---