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Andrew Newberg, MD

Thyroid ScinTigraphy

1. What patient preparation is required before thyroid scanning?

Patients should fast after midnight or, at a minimum, for 4 hours before receiving the radioactive iodine (I)-123 pill to

maximize absorption through the gut. Patients should also avoid ingesting foods containing high amounts of iodine

because they would saturate the iodine stores in the body and thyroid and diminish the uptake of the radioactive iodine.

Foods such as shellfish and seaweed and supplements containing large amounts of iodine should be avoided. Patients

should also discontinue taking any medication that would affect thyroid function, particularly thyroid replacement

medication (for at least 2 weeks); thyroid-blocking medication (propylthiouracil or methimazole for at least 5 days); and

other medications, such as amiodarone, that contain substantial amounts of iodine. Finally, patients should not undergo

scanning if they have received iodinated contrast agents as recently as 1 month before the scan (although sometimes

the effects of such contrast agents on the thyroid can be seen 3 months later).

2. Which isotopes can be used for imaging the thyroid, and how do they compare

physiologically?

The most common radioactive materials used in thyroid imaging are the various isotopes of iodine. I-123 is the most

commonly used isotope because of its excellent image quality and relatively low radiation exposure. Isotopes I-125 and

I-127 are not typically used because of their high radiation exposure and poorer image quality. I-131 is still used for

thyroid imaging in patients with thyroid cancer. Its higher photon energy theoretically makes it easier to detect cancer

foci in the deep soft tissues in the body. I-131 releases a higher radiation exposure to patients, however, and the image

quality is not typically as good as I-123. All of the iodine tracers are trapped by the thyroid and undergo organification

into the thyroid hormones. The other primary tracer for thyroid imaging is technetium (Tc)-99m pertechnetate. This

material is trapped (not organified) by the thyroid and can provide high-quality images. The amount of uptake of the

Tc-99m tracer cannot be used to plan I-131 therapy, however, as is the case with the iodine tracers.

3. What are normal thyroid scan and iodine uptake results?

Thyroid images are obtained 24 hours after the administration of I-123 or approximately 30 minutes after Tc-99m.

The gland should appear uniform throughout with smooth contours. The isthmus and pyramidal lobes can be observed,

although they are more commonly seen in patients with Graves disease. Iodine uptake values are usually obtained at

2 and 24 hours after administration of the I-123 pill. The 2-hour uptake should be 2% to 10%, and the 24-hour uptake

is usually 10% to 30%. These values may differ slightly, depending on the uptake probe that is used, so it is always

important to know what a particular institution considers to be the normal range (

Fig. 56-1

).

4. Can a patient with normal I-123 uptake still have Graves disease?

The iodine uptake refers to the activity of the gland and not to whether the patient is hyperthyroid. Although a patient

may have normal scan results and uptake values, if the thyroid function tests indicate that the patient is hyperthyroid,

a normal iodine uptake is still too high. A hyperthyroid patient should have a gland that is almost completely shut down.

An uptake of 20% to 30% is too high if a patient is hyperthyroid with an undetectable thyroid-stimulating hormone (TSH)

level. In such a setting, a patient with normal scan results can still have Graves disease.

5. What are possible outcomes after I-131 therapy for Graves disease?

There are three possible outcomes:

•

A patient may be hypothyroid if the dose is sufficient to destroy most of the thyroid tissue and function.

•

A patient may be euthyroid if the dose diminishes the function of the thyroid, but there is sufficient activity to maintain

normal circulating levels of thyroid hormone.

•

A patient may be undertreated (i.e., remain hyperthyroid), with the gland continuing to produce high levels of thyroid

hormone.

In the hypothyroid case, patients typically begin undergoing thyroid replacement therapy. In the hyperthyroid case,

patients usually require retreatment with a higher dose of I-131. The likelihood of each outcome depends on the

range of doses given. Lower doses result in fewer hypothyroid patients, with more patients requiring retreatment for

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Thyroid ScinTigraphy

persistent hyperthyroidism. Higher doses result in more

patients being hypothyroid, and fewer needing to be

retreated.

6. How is a patient with a hot nodule

treated for hyperthyroidism?

Hot nodules are typically treated with I-131

therapy, but are more resistant. It is usually

recommended that the dose be higher than that

used for treating a patient with Graves disease.

The overall approach to therapy is similar,

however, including the preparation and radiation

precautions that are followed after the dose is

given.

7. Why is a patient with a hot nodule

more likely to end up euthyroid?

Patients with hot nodules have a higher likelihood

of ending up euthyroid because the hot nodule

typically causes a suppression of the remainder

of the gland. This suppression protects the

gland from I-131 therapy. When the nodule is

eliminated by I-131, the patient’s thyroid function

decreases, and eventually the TSH level begins

to increase. This increased TSH stimulates the

remaining thyroid tissue to begin producing

thyroid hormone again. Because most of the

suppressed tissue was protected, the patient

may be able to make normal amounts of thyroid

hormone (

Fig. 56-2

).

8. What are the typical radiation safety

precautions that patients must

follow after I-131 therapy?

For approximately 5 days, patients treated with

I-131 must do the following: sleep alone, if

possible; avoid kissing and sexual intercourse;

minimize time with pregnant women and young

children; minimize close contact with others;

use good hygiene habits; wash hands thoroughly

after each toilet use; use a separate bathroom,

if possible; flush the toilet two times after each

use; drink plenty of liquids; use disposable eating

utensils; use separate bath linens; launder linens

and underclothing separately initially; maintain

a toothbrush in a separate holder; if possible,

not prepare food for others; not apply cosmetics

or lip balm; and wipe the telephone mouthpiece

with a tissue after each use. In addition,

nursing women should completely discontinue

breastfeeding until their next child, and all

women should not attempt to become pregnant

for at least 90 days.

9. When should a patient with multinodular goiter be treated with I-131?

Typical indications are either symptomatic hyperthyroidism or symptomatic mass effect related to the enlarged thyroid.

Hyperthyroid patients can usually be treated with I-131, although with usually two to three times the dose as that for a

patient with Graves disease because the multinodular gland can be more refractory to the effects of I-131. Patients with

compression of the airway or other vital structures in the neck or chest from an enlarged thyroid can be treated either

surgically or with I-131. I-131 therapy is much less invasive and usually results in substantial shrinkage of the gland,

which may obviate the need for surgery. Some clinicians are concerned, however, that potential inflammatory response

from high levels of I-131 may result in clinical worsening (

Fig. 56-3

).

2 HR I-123 ANTERIOR

RAO 24 HR

LAO 24 HR

24 HR ANTERIOR

MARKER

Figure 56-2.

Thyroid scan shows large “hot” nodule in left lobe

with suppression of right lobe (clockwise from top left : anterior view,

anterior view with marker in sternal notch, right anterior oblique view,

and left anterior oblique view).

Anterior

Anterior Marker 128 x 128

Figure 56-1.

Normal thyroid scan (clockwise from top left : anterior

view, anterior view with marker in sternal notch, right anterior oblique

view, and left anterior oblique view).

Thyroid ScinTigraphy

391

nuclear radiology

10. What factors affect the likelihood that a cold nodule represents thyroid cancer?

Several factors relating to patient demographics, patient history, and features of the nodule affect the probability that

a cold nodule may harbor cancer. Younger male patients with cold nodules are more likely to have cancer than older

female patients with similar findings. Exposure to radiation in the neck is an important risk factor for cancer. Cold

nodules in the setting of a multinodular goiter are substantially less likely to be cancer than other cold nodules. Finally,

ultrasound findings of mixed cystic and solid components within a cold nodule are also more suggestive of thyroid

cancer (

Fig. 56-4

).

11. Why can I-131 be used to treat thyroid cancer if the original nodule is cold?

Although a cold nodule does not appear to take up any of the radioactive iodine on scintigraphy, its inactive appearance

is in comparison to the background activity of the normal gland. Thyroid cancer cells generally take up the iodine, but

not as well as normal tissues. For this reason, thyroid cancer appears cold on the scan, but it can still take up enough

iodine for therapy to be effective. To enhance the therapeutic effects of I-131 therapy, the normal thyroid is surgically

removed. By eliminating much of the normal thyroid tissue, there is a better chance for any remaining thyroid cancer

cells to take up I-131. The hope is that by removing the thyroid and keeping the patient off thyroid replacement

hormones, the TSH level would be greater than 30 IU/L, which should maximally stimulate any thyroid cancer cells to

take up I-131. Patients are also usually prescribed a low-iodine diet to “starve” any thyroid cells, maximizing uptake of

I-131 further.

12. What foods should be avoided as part of a low-iodine diet?

A low-iodine diet is highly restrictive because many foods contain iodine. Foods to be avoided include most dairy

products (milk, yogurt, ice cream, cheese), luncheon meats, bacon, hot dogs, fish, shellfish, noodles, pasta, cereals,

pastry, breads, packaged rice, canned juices, canned fruits, canned or frozen vegetables, cocoa mixes, diet bars, jams/

jellies, nuts, mustard, olives, candy, pretzels, and any snack foods. Patients can eat small portions fresh chicken, fresh

potatoes, fresh fruit, all fresh vegetables except spinach, sweet butter, vegetable oils, onion or garlic powder, fresh

herbs, and popcorn with no salt.

13. What is the general management plan for patients diagnosed with thyroid cancer?

Usually, patients who are diagnosed with thyroid cancer initially undergo surgery to remove the gland. Occasionally,

one lobe is removed first, and then, if cancer is detected, the other lobe is removed. Most of the time, when cancer has

already been definitively diagnosed with biopsy, as much of the gland is removed as possible. The patient goes to the

nuclear medicine department for a scan and therapy. The pretherapy scan usually shows some uptake in the thyroid bed

because it is difficult to remove all of the gland surgically, especially when it is adjacent to critical structures in the neck.

Therapy is performed using I-131, given as an oral dose. The purpose of this therapeutic dose is to ablate any remaining

normal thyroid tissue (some of which is usually left by the surgeon because of its proximity to critical structures in

the neck) and to eliminate any remaining thyroid cancer cells. Patients are followed up at regular intervals with neck

and whole-body iodine scans and serum measures of thyroglobulins. Patients can be retreated with I-131 if there is

a recurrence (

Fig. 56-5

).

Anterior

Anterior Marker 128 x 128

Figure 56-4.

Thyroid scan shows large cold nodule in the right pole

(clockwise from top left: anterior view, anterior view with marker in

sternal notch, right anterior oblique view, and left anterior oblique view).

ANT MARKER

Figure 56-3.

Thyroid scan shows multinodular goiter with

numerous hot and cold regions (clockwise from top left: anterior

view, anterior view with marker in sternal notch, right anterior

oblique view, and left anterior oblique view).

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Thyroid ScinTigraphy

14. What does “stunning” mean with regard to I-131 scanning?

Several studies have suggested that a typical I-131 imaging dose of 4 mCi actually has a small therapeutic effect on the

thyroid and “stuns” the thyroid cells, making them slightly resistant to the larger therapeutic dose. For this reason, some

clinicians have recommended using I-123 for whole-body imaging before I-131 therapy to avoid this effect.

15. What range of doses of I-131 is typically recommended for therapy for thyroid cancer

in patients with the scans in

Figs. 56-6

and

56-7

?

The range of doses given to patients being treated for thyroid cancer depends on the extent of the cancer and the

institution. Either way, there is usually a graded scale of doses that increases for worsening extent of disease.

16. How does a recombinant human thyrotropin alfa (Thyrogen) scan work?

A Thyrogen scan is usually performed in the second year of follow-up for patients treated with I-131 for thyroid

cancer. A Thyrogen scan is performed by subcutaneously administering a synthetic version of TSH on 2 consecutive

days and then performing the iodine scan to detect thyroid tissue. Usually, patients stop taking thyroid medications to

ensure that the TSH is maximally stimulating any residual thyroid tissue so that it can be detected on the scan. By giving

the patient TSH, a similar physiologic state is obtained.

Figure 56-6.

Anterior view of I-123

whole-body scan of a patient with thyroid

cancer after surgical resection. The scan

of this patient who is now presenting for

I-131 therapy shows multiple foci in the

neck, which are consistent with lymph node

involvement.

Figure 56-5.

Anterior view of I-123 whole-

body scan of a patient with thyroid cancer after

surgical resection. The scan of this patient who is

now presenting for I-131 therapy shows two foci

in the neck of residual thyroid tissue. The activity

in the nasopharyngeal area, gut, and bladder

represents areas of normal excretion of I-123.

Key Points: Typical Dose Ranges for I-131

Therapy for Thyroid Cancer

1. 60 to 100 mCi for low-risk disease

2. 125 mCi for lymph node involvement

3. 150 mCi for lung involvement

4. 175 mCi for bone involvement

Thyroid ScinTigraphy

393

nuclear radiology

17. What are possible causes of a

thyroid scan in which there is no

uptake or minimal uptake by the

thyroid?

When there is no uptake observed on a scan, there

are several potential causes. Patients may have

thyroiditis, in which the inflammatory process has

destroyed most functioning tissue. Such patients

can be hyperthyroid or hypothyroid. Other causes

of minimal uptake in the thyroid gland include

exogenous sources of thyroid hormone (some

patients take thyroid hormone as a form of diet

pill) or iodine (e.g., recent iodinated contrast agent

administration, ingestion of drugs containing

iodine such as amiodarone, consumption of high-

salt or seafood diets, and ingestion of alternative

medicine supplements). Exogenous sources of

thyroid tissue, such as struma ovarii (thyroid tissue

in a monodermal ovarian teratoma), should also be

considered in such patients.

18. How is fluorodeoxyglucose positron

emission tomography (FDG PET)

useful in the management of thyroid

cancer?

In general, PET has no application in the initial

diagnosis of thyroid carcinoma because there

is overlap of FDG uptake between benign and

malignant thyroid tumors. PET can be very useful,

however, in patients previously treated for thyroid

cancer who now have increasing tumor markers and an I-131 scan with negative results. FDG PET can be a useful

adjunct imaging modality to help detect sites of tumor recurrence. The sensitivity for detection of recurrence with

FDG PET is improved with hormonal withdrawal or administration of recombinant TSH. FDG PET also has a role in the

management of patients with anaplastic, Hürthle cell, or medullary carcinoma because these tumors are not iodine

avid.

B

iBliography

[1] N.S. Alnafisi, A.A. Driedger, G. Coates, et al., FDG PET of recurrent or metastatic

131

I-negative papillary thyroid carcinoma, J. Nucl. Med.

41 (2000) 1010–1015.

[2] H.J. Dworkin, D.A. Meier, M. Kaplan, Advances in the management of patients with thyroid disease, Semin. Nucl. Med. 25 (1995) 205–220.

[3] W.D. Leslie, A.C. Peterdy, J.O. Dupont, Radioiodine treatment outcomes in thyroid glands previously irradiated for Graves’ hyperthyroidism,

J. Nucl. Med. 39 (1998) 712–716.

[4] L.K. Shankar, A.J. Yamamoto, A. Alavi, et al., Comparison of I 13I scintigraphy at 5 and 24 hours in patients with differentiated thyroid

cancer, J. Nucl. Med. 43 (2002) 72–76.

[5] P.A. Singer, D.S. Cooper, G.H. Daniels, et al., Treatment guidelines for patients with thyroid nodules and well differentiated thyroid cancer,

Arch. Intern. Med. 156 (1996) 2165–2172.

Figure 56-7.

Anterior and posterior views of I-123 whole-body scan

of a patient with thyroid cancer after surgical resection who is now

presenting for I-131 therapy. The scan shows multiple foci of intense

activity in the neck, mediastinum, thorax, abdomen, and right humerus.

These findings suggest widespread disease with bone involvement.