How much Tc-99m given for thyroid scan?

Answer 1

10 mCi of Tc-99m is the dosage required for a thyroid scan

ENDOCRINE NUCLEAR MEDICINE

THYROID SCANINDICATIONS:

1. Determination of thyroid size, function, and position.
2. Evaluation of functional status of thyroid nodules.
3. Evaluation of thyroid and neck masses.
4. Evaluation of patients with history of head and neck irradiation.
5. Quantitative thyroid uptake (I-131 uptake).
6. Detection of ectopic thyroid tissues such as substernal or sublingual locations of thyroid tissue (I-123).
7. Treatment for hyperthyroidism, neoplasm (I-131).
8. Detection of thyroid metastases and assessment of response to therapy (I-131).

PATIENT PREPARATION:

1. Patients are kept NPO for at least three hours before uptake studies. No fasting is required for Technetium scanning.
2. Patients should be off iodine-containing drugs. This includes radiographic contrast! The minimum time interval between prior iodine administration and scanning varies from one week (e.g. iodine solution) to 3 weeks (Synthroid) to 2-10 years (oil-based Myelogram). If the patient has an IVP or a CT with IV contrast, a thyroid scan should not be attempted for 4-6 weeks. Many other medications interfere with scanning (PCN, antihistamines, etc.). Consult Nuclear Medicine for a complete list.

RADIOPHARMACEUTICAL:

1. For Iodine Uptake 10-50 mCi of I-131 p.o. (uptake alone).
2. For Technetium Pertechnetate 10 mCi Tc-99m pertechnetate given I.V.
3. Metastatic Workup 5 mCi I-131 p.o.
4. Therapy 5-30 mCi I-131 p.o. for hyperthyroidism.

75-300 mCi I-131 p.o. for cancer treatment.

The critical organ with iodine radiopharmaceuticals is the thyroid, which receives 1-2 rad from a typical I-123 uptake and scan. Doses for I-131 are considerably higher; I-131 ablation therapy dose, over 30 mCi, require hospitalization for proper isolation. 99mTcO4 gives 0.12-0.20 rad/mCi to the thyroid.

SCANNING TIME:For 99mTcO4 scanning is begun approximately 20 minutes after injection.

For metastatic workup - 48 and 96 hours post oral administration.

For thyroid uptake test -- 24 hours.

OTHER CONSIDERATIONS:

1. In some institutions, I-123 is used for uptake and scan; I-131 is reserved for metastatic workup. I-131 is also used for treatment of hyperthyroidism and thyroid cancer. 99mTcO4 has the advantage of short time to scan and avoidance of second trip to the department, however, it cannot be used for evaluating thyroid uptake. Hot nodules with 99mTcO4 may require evaluation with iodine.
2. Both agents (Tc, I) cross the placenta and are secreted in breast milk. Pregnancy is, therefore, a relative contraindication and breast feeding must be interrupted if scanning is necessary for 7 days with Tc-99m and should be terminated when using I-123 or I-131 (for that particular pregnancy).
3. Iodine-containing preparations as well as several non-Iodine medications interfere with both 99mTcO4 and Iodine scans, Please consult the Nuclear Medicine Department prior to scanning. (See "patient preparation").

RATIONALE:Iodine is actively trapped, concentrated and organified by the thyroid gland. This is the basis for morphologic and functional imaging. The uptake study measures the percentage of the administered dose localizing in the gland at a fixed time; this reflects gland function. Technetium is also trapped by the thyroid, but is not organified. It washes out of the gland after peaking at approximately 20 min post injection. Therefore, it is not ideal for evaluating percent uptake. Normal range for 24 hour I-131 (and I-123) thyroid uptake is 7 to 33% in our laboratory.