Thyroid Withdrawal Scan

Physiological basis of thyroid withdrawal scan

The pathophysiological basis of the radioactive iodine scan using the withdrawal protocol is based on the preferential uptake of radioactive iodine (RAI) by thyroid cells due to their unique ability to concentrate iodine. This property is used to image the thyroid gland and diagnose various thyroid conditions, such as hyperthyroidism, thyroid cancer, and thyroid nodules[1].

The withdrawal protocol in the context of the management of differentiated thyroid cancer requires discontinuation of thyroid hormone replacement therapy, specifically levothyroxine, for a period of time prior to the scan, typically 4-6 weeks[2]. This protocol aims to increase the levels of thyroid-stimulating hormone (TSH) in the blood. Elevated TSH levels (typically >30mIU/ml) stimulate the thyroid gland to absorb more iodine, enhancing the effectiveness of the RAI scan[2].

When the patient undergoes the RAI scan, a small dose of radioactive iodine (I-123 or I-131) is administered orally. Thyroid cells absorb radioactive iodine, and a gamma camera captures images of the distribution of the radioactive substance in the thyroid gland[3]

Mechanism of action of I-123 and I-131

Iodine-131 (I-131) and Iodine-123 (I-123) are radioactive iodine isotopes used in nuclear medicine for diagnostic and therapeutic purposes, particularly to assess thyroid function[4].

I-131 is a beta and gamma-emitting isotope with an eight-day half-life, used primarily to diagnose and treat hyperthyroidism and thyroid cancer[5]. It is absorbed by the thyroid gland similarly to non-radioactive iodine, allowing evaluation of thyroid function and visualization of the structure of the gland. Due to the longer half-life and increased radiation exposure, I-131 has limitations for some diagnostic applications[6].

I-123, a gamma-emitting isotope with a 13-hour half-life, is more suitable for certain diagnostic purposes than I-131 due to its lower radiation exposure[7]. I-123 is not used for therapeutic purposes, as it lacks beta particle emissions necessary to destroy thyroid cells[8]

Practice Guide for RAI scans

Interpretation of Radioactive Iodine Scans

Imaging findingsManagement plan
Absent radioiodine uptake in the thyroid bedSuboptimal TSH stimulation Poor adherence to a low-iodine diet
Increased focus of uptake in the thyroid bedPossible excess remnant tissue or persistent thyroid cancer. The patient may require repeat surgery or fine needle biopsy
Focus of uptake outside the thyroid bedAdditional testing is required. Evaluate metastatic disease and consider dosimetrically guided instead of empiric I-131 activities for treatment

Based on reference[9]

Clinical Trial Evidence

This study investigated the use of human recombinant thyrotropin (rhTSH) as an alternative to the withdrawal of thyroid hormone for radioiodine detection in patients with differentiated thyroid cancer. The research included 127 patients who underwent whole-body radioiodine scanning using both methods. The results showed that the rhTSH-stimulated scans were equivalent to thyroid hormone withdrawal scans in 66% of the patients with positive scans, superior in 5%, and inferior in 29%. When considering all patients, both methods were equivalent in 83% of cases. Patients experienced fewer symptoms of hypothyroidism and mood disturbances with rhTSH administration. Although rhTSH scanning has lower sensitivity, it offers a viable alternative to the withdrawal of thyroid hormone with reduced side effects[10].

References

1.        Iqbal, A., and Rehman, A. (2023) Thyroid Uptake and Scan, in StatPearls, StatPearls Publishing, Treasure Island (FL).

2.        Haugen, B.R., Alexander, E.K., Bible, K.C., Doherty, G.M., Mandel, S.J., Nikiforov, Y.E., Pacini, F., Randolph, G.W., Sawka, A.M., Schlumberger, M., Schuff, K.G., Sherman, S.I., Sosa, J.A., Steward, D.L., Tuttle, R.M., and Wartofsky, L. (2016) 2015 American Thyroid Association Management Guidelines for Adult Patients with Thyroid Nodules and Differentiated Thyroid Cancer: The American Thyroid Association Guidelines Task Force on Thyroid Nodules and Differentiated Thyroid Cancer. Thyroid, 26 (1), 1–133.

3.        Hänscheid, H., Lassmann, M., Luster, M., Thomas, S.R., Pacini, F., Ceccarelli, C., Ladenson, P.W., Wahl, R.L., Schlumberger, M., Ricard, M., Driedger, A., Kloos, R.T., Sherman, S.I., Haugen, B.R., Carriere, V., Corone, C., and Reiners, C. (2006) Iodine Biokinetics and Dosimetry in Radioiodine Therapy of Thyroid Cancer: Procedures and Results of a Prospective International Controlled Study of Ablation After rhTSH or Hormone Withdrawal. Journal of Nuclear Medicine, 47 (4), 648–654.

4.        Silberstein, E.B. (2012) Radioiodine: the classic theranostic agent. Semin Nucl Med, 42 (3), 164–170.

5.        Galasko, G.T. (2017) 29 – Pituitary, Thyroid, and Parathyroid Pharmacology, in Pharmacology and Therapeutics for Dentistry (Seventh Edition) (eds.Dowd, F.J., Johnson, B.S., and Mariotti, A.J.), Mosby, pp. 417–428.

6.        Wyszomirska, A. (2012) Iodine-131 for therapy of thyroid diseases. Physical and biological basis. Nucl Med Rev Cent East Eur, 15 (2), 120–123.

7.        Kim, P.D., and Tran, H.D. (2023) I-123 Uptake, in StatPearls, StatPearls Publishing, Treasure Island (FL).

8.        Mettler, F.A., and Guiberteau, M.J. (2012) 4 – Thyroid, Parathyroid, and Salivary Glands, in Essentials of Nuclear Medicine Imaging (Sixth Edition) (eds.Mettler, F.A., and Guiberteau, M.J.), W.B. Saunders, Philadelphia, pp. 99–130.

9.        Van Nostrand, D. (2019) Radioiodine Imaging for Differentiated Thyroid Cancer: Not All Radioiodine Images Are Performed Equally. Thyroid, 29 (7), 901–909.

10.      Ladenson, P.W., Braverman, L.E., Mazzaferri, E.L., Brucker-Davis, F., Cooper, D.S., Garber, J.R., Wondisford, F.E., Davies, T.F., DeGroot, L.J., Daniels, G.H., Ross, D.S., and Weintraub, B.D. (1997) 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, 337 (13), 888–896.

Kindly Let Us Know If This Was helpful? Thank You!

About the Author MyEndoConsult

The MyEndoconsult Team. A group of physicians dedicated to endocrinology and internal medicine education.

{"email":"Email address invalid","url":"Website address invalid","required":"Required field missing"}
>