Introduction
Thethyroid uptake and scan is a radiologic diagnostic tool used to determine thyroid function and pathologies. This procedure works on the principle of the unstable nuclide of the atom, which tries to attain stability by releasing alpha, beta, and gamma rays. Clinicians use these rays for diagnostic and treatment purposes.
The test uses a radioactive tracer, a protein, or a molecule attached to radioactive material. The radioactive tracer is administered to the patient, and a probe measures the amount of iodine uptake by the thyroid gland. The thyroid scan checks for the even spread of the tracer in the gland. More than 20 radioactive tracers exist, of which2 isotopes of iodine I-123 and I-131 and 99m-technetium pertechnetate are the usual agents used in this test. I-131 has a longer half-life than I-123, so I-123 is used more frequently than I-131 due to less radiation exposure to the body. 99m-Technetium pertechnetate is an analog of iodine, so it gets transported to the thyroid gland similarly to iodine.111In-pentoxide, thallium-201 (201T1),99mTc-sestamibi, and 99mTc-tetrofosmin are some of the less commonly used radioactive tracers in this test.[1]This test is different than radioactive iodine therapy, which treats thyroid cancers.
Procedures
Clinicians usually perform the thyroid uptake and scan in the outpatient setting. The patient should be prepared well for this, as certain foods and medications interfere with radiotracer uptake in the thyroid gland. The patient can be allergic to the radiotracer, so careful evaluation is necessary. If a patient is taking antithyroid medications such as methimazole or propylthiouracil, they should receive instructions to holdthese medicationsfor at least5 days. Before the test, prior studies of the thyroid gland and baseline TSH and free T4 levels are necessary. In thyroid cancer patients, thyroglobulin and thyroglobulin antibodies may also need to be checked.
Radiotracer is given to the patient intravenously or orally. The timing of radiotracer administration before the scan varies according to the route. The patients are asked to take an oral radiotracer 24 hours before the scan or be injected with a radiotracer 30 minutes before the scan. Sodium iodide symporters located on the thyroid gland cells are responsible for uptaking radioactive iodine. The patient then lies on the movable examination table, and the gamma camera takes serial images of the thyroid gland from3 different angles. The patient should lie still while capturing images. The clinical history, thyroid examination, and thyroid function tests all merit consideration while interpreting the results of this test.[2]
Indications
The indications of thyroid uptake and scan are as follows:
Differential diagnosis of hyperthyroidism
Suspected thyroid cancer
Suspected metastasis of thyroid cancer
Thyroid nodule
Thyroid inflammation
Determine the efficacy of radioactive iodine therapy
Organification (incorporation of iodine into thyroglobulin) defects
Determine congenital thyroid defects[3][4][5]
Potential Diagnosis
The thyroid uptake and scan have proved to help diagnose the following diseases:
Graves disease.
Toxic nodular goiter
Toxic adenoma
Thyroiditis
Congenital defect of thyroid hormone synthesis
Iodine deficiency
The recovery from subacute, silent, or postpartum thyroiditis
Destructive thyroiditis
Subacute thyroiditis
Silent thyroiditis
Postpartum thyroiditis[6]
Normal and Critical Findings
The normal thyroid uptake of radiotracer is 3to 16% at 6 hours and 8to 25% at 24 hours. These values may change according to laboratory standard techniques or patient dietary habits.
The thyroid gland can uptake more or less radioactive iodine than normal. More than normal uptake indicates a hyperactive thyroid, and less than normal uptake infers a hypoactive thyroid gland or interference with the uptake (see the list of potential conditions described below).
The following are some of the causes of increased uptake of radiotracer:
Hyperthyroidism due to Graves, multinodular goiter, or thyroid adenoma
Goiter
Early-stageof Hashimoto thyroiditis
Iodine deficiency
The recovery phase from subacute, silent, or postpartum thyroiditis
Pregnancy
Lithium carbonate therapy
Withdrawal of antithyroid medication
Rebound after the suppression of thyrotropin
Congenital defects of thyroid hormone synthesis
Some of the causes of decreased uptake of radiotracer are as follows:
Primary hypothyroidism
Central hypothyroidism
Destructive thyroiditis
Subacute thyroiditis
Silent thyroiditis
Postpartum thyroiditis
Excess iodine
Dietary supplements
Radiological contrast
Medications
Amiodarone
Antithyroid drugs
Perchlorate
Thiocyanate
Sulphonamides
Sulphonylurea
High-dose glucocorticosteroids
Topical iodine
See AlsoThyroid Function TestsPost-thyroidectomy
External neck radiation
Interfering Factors
Factors that clinicians shouldconsider before conducting thyroid uptake and scan are as follows:
Diarrhea that can decrease the absorption of the dye if given orally
Head CT with oral or intravenous contrast within the past2 weeks
Unacceptable quantity of iodine in the diet
Hypochloremia that can increase the absorption of the radiotracer
Iodine-containing drugs (amiodarone)
Thyroid hormone replacement and antithyroid drugs
Chronic renal failure that impairs iodide clearance expands the iodide pool and lowers the %RAIU[7]
Complications
Radioactive iodine uptake and scan is a safe procedure. Some of the complications of this procedure are as follows:
Pain at the injection site
Hypersensitivity and anaphylaxis to radiotracer
Exposure of the fetus or baby, if performed during pregnancy or lactating period[8]
Patient Safety and Education
The following precautions are necessary before performing a thyroid uptake scan:
Pregnancy should be ruled out with either serum or urine pregnancy tests, and the clinician should ask the patient to avoid pregnancy for6 months after radioactive iodine administration.
The patient should avoid breastfeeding after performing this test as radioactive iodine can be secreted in breast milk.
A trained professional should administer this test.
The patient should receive counseling regarding physical contact safety measures, including avoiding the exposure of urine, stool, saliva, vomit, blood, and body fluids, as well as perspiration, for 48 hours.
Patients should also avoid public transportation and sitting close to others.
Patients should carry documents indicating the date, provider information, and radionuclide used while traveling through the port of entry within4 months of the procedure.
Patients should receive counseling on flushing twice after urinating or defecating for 24 to 48 hours after the procedure.
Clinical Significance
The thyroid uptake and scan play a central role in diagnosing thyroid diseases and abnormalities in thyroid function as it provides detailed information on the functions and anatomy of the thyroid gland. Different radioactive tracers function to detect or diagnose various thyroid diseases. Iodine,technetium pertechnetate, gallium-67, andfluoro-deoxy-glucose are all used. Relative uptake ofiodine and technetium pertechnetate by focal thyroid nodule is labeled aswarm,cold, or hot. Other uses ofiodine includetreatment of thyroid cancer,evaluation of residual/recurrent disease, evaluation of distant metastasis, and follow-up of patients after thyroidectomy.[9][10]
References
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Tran P, Desimone S, Barrett M, Bachrach B. I-131 treatment of graves' disease in an unsuspected first trimester pregnancy; the potential for adverse effects on the fetus and a review of the current guidelines for pregnancy screening. Int J Pediatr Endocrinol. 2010;2010:858359. [PMC free article: PMC2838222] [PubMed: 20300595]
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Ladenson PW, Braverman LE, Mazzaferri EL, Brucker-Davis F, Cooper DS, Garber JR, Wondisford FE, Davies TF, DeGroot LJ, Daniels GH, Ross DS, Weintraub BD. 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 Sep 25;337(13):888-96. [PubMed: 9302303]
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American Thyroid Association Taskforce On Radioiodine Safety. Sisson JC, Freitas J, McDougall IR, Dauer LT, Hurley JR, Brierley JD, Edinboro CH, Rosenthal D, Thomas MJ, Wexler JA, Asamoah E, Avram AM, Milas M, Greenlee C. Radiation safety in the treatment of patients with thyroid diseases by radioiodine 131I : practice recommendations of the American Thyroid Association. Thyroid. 2011 Apr;21(4):335-46. [PubMed: 21417738]
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Broos WAM, Wondergem M, van der Zant FM, Knol RJJ. Dual-Time-Point 18F-Fluorocholine PET/CT in Parathyroid Imaging. J Nucl Med. 2019 Nov;60(11):1605-1610. [PubMed: 30877179]
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Arciero CA, Shiue ZS, Gates JD, Peoples GE, Dackiw AP, Tufano RP, Libutti SK, Zeiger MA, Stojadinovic A. Preoperative thyroid ultrasound is indicated in patients undergoing parathyroidectomy for primary hyperparathyroidism. J Cancer. 2012;3:1-6. [PMC free article: PMC3245602] [PubMed: 22211139]
Disclosure: Aqsa Iqbal declares no relevant financial relationships with ineligible companies.
Disclosure: Anis Rehman declares no relevant financial relationships with ineligible companies.
