Thyroid Flashcards
radionuclides transported in the thyroid gland
I-123, I-131, Tc-99m
Which radionuclide(s) are transported into the thyroid but not organified?
Tc-99m
What is special about Tc-99m in the thyroid
transported but not organified
I-131 energy
365 keV
I-131 half life
8 days
8 day half life
I-131
365 keV
I-131
13 hour half life
I-123
I-123 half life
13 hours
159 keV
I-123
I-123 energy
159 keV
Pt with recent iodinated contrast and Jod-Basedow phenomenon comes in for imaging of the thyroid. What imaging agent will you give them?
Tc-99m-O4. (pertechnetate)
TYPICALLY you would choose I-123 except in scenarios where the thyroid won’t take up iodine normally –> aka thyroid blocker or recent iodinated contrast
you can resume breast feeding in 24hr with what thyroid imaging radiotracer?
Tc-99m
you can resume breast feeding in 48hr with what thyroid imaging radiotracer?
I-123
you can NEVER resume breast feeding with what thyroid radionuclide?
I-131
Breast feeding trivia for :
Tc99m, I-123, I-131
Tc99m: resume in 24 hr
I-123: resume in 48 hour
I-131: NEVER for the current baby
How much I-131 or I-123 is given for iodine uptake tests?
5 mCi of I-131
or
10-20 mCi of I-123
What are the time points at which thyroid uptake is measured on an iodine uptake test?
4-6 hours AND 24 hrs
Normal % uptakes in iodine uptake test
5-15% (4-6 hr)
10-30% (24 hr)
the 4 hr is doubled to remember the 24 hr range
medications that decrease thyroid uptake
thyroid blockers, nitrates, IV contrast, AMIODARONE
How does kidney function affect thyroid uptake
reduced kidney function –> increases stable iodine pool (decreased excretion) –> decreased uptake
high T3 and T4, low TSH, diffuse thyroid uptake
Graves disease
Situations in which thyroid will take up Tc but NOT Iodine on 24 hr imaging
- congenital enzyme deficiency that inhibits organification
- drug like PTU that blocks organification
- Jod-Basedow phenomenon/recent IV contrast administration
Most common subtype of thyroid cancer
papillary
Factors that make something in the thyroid resistant to treatment with I-131
- Medullary Subtype CA (does not drink the tracer since it is neuroendocrine origin)
- History of prior I-131 (“easy gland has been killed off”) –> have to use about 50% more during re-treatment than the original dose
- History of Methimazole treatment (Even if years ago)
Medullary subtype CA association
MEN 2
medullary subtype is neuroendocrine in origin
Thyroid cancer that takes up MIBG or octreotide
Medullary subtype (neuroendocrine in origin)
thyroid cancer that is neuroendocrine in origin
Medullary subtype CA
MEN2 associated thyroid cancer
Medullary subtype
Ideal TSH for treating thyroid cancer after surgical excision
> 30 (50 is ideal)
stop giving thyroid hormone or give recombinant TSH “thyrogen”`
why does I-131 deliver 100 times more dose to the thyroid per mCi than I-123?
beta emission
True or false: Thyroid uptake can be seen with free technetium.
True. In fact, Tc-pertechnetate is one agent that can be used for certain types of thyroid
imaging. Remember that classic regions for free technetium uptake include salivary glands,
thyroid and gastric uptake.
When might you consider using Tc over iodine for thyroid imaging?
One scenario where Tc-99m may be superior to I123/I131 for thyroid imaging is for patients
who are on thyroid blockade including patients who have recently received iodinated contrast
as the thyroid may not take up the radioiodine but would take up the Tc-99m.
for patients
who are on thyroid blockade what imaging agent should you consider?
Tc-99m pertectnetate
What is the difference between trapping and organification in the thyroid?
Trapping refers to a radiotracer getting transported into the thyroid gland. This is the initial
entry of iodine and other tracers into the thyroid. Organification refers to the iodine actually
getting oxidized by thyroid peroxidase and then binding to a tyrosyl moiety, essentially making
it so the iodine will not wash out of the thyroid.
What is the normal arterial supply of the thyroid?
The thyroid is supplied by the superior thyroid artery and the inferior thyroid artery. The superior thyroid artery is a branch of the external carotid artery and the inferior thyroid artery is a branch of the thyrocervical trunk.
If a patient presents with recent upper respiratory illness presenting with acute neck pain in
the region of the thyroid and symptoms of hyperthyroidism, what is the top differential
consideration?
A classic history for De Quervain thyroiditis aka subacute granulomatous thyroiditis is
presentation with acute neck pain and symptoms of hyperthyroidism to include tachycardia,
palpitations, and hot flushes. The cause is thought to be post-viral inflammation of the thyroid
following an upper respiratory infection and is most common in middle aged females.
What is the normal course for thyroid hormone levels during the evolution of De Quervain thryoiditis?
First, thyrotoxicosis as the thyroid gland is inflamed and releases thyroid hormone into the
bloodstream. Second, hypothyroidism as the thyroid becomes depleted of normal thyroid
hormone. Finally, a return to the euthyroid state for most patients.
Is De Quervain’s thyroiditis treated with radioactive iodine?
No. This is a self-limiting disease and one would not want to ablate the thyroid that is expected
to recover on its own. Also, the hyperthyroidism is transient and self-resolving and thus does
not require radioiodine ablation.
What are extrathryoidal manifestations of Graves disease?
Extrathryroidal manifestations of Graves include Graves ophthalmopathy (remember proptosis
with orbital extraoccular muscular enlargement (IMSLO) in order of higher frequency first
involving the inferior rectus, medial rectus, superior rectus, lateral rectus and lastly oblique
muscles), pretibial myxedema/thyroid dermopathy, thyroid acropachy (finger swelling,
periosteal reaction, etc)., and classic symptoms of hyperthyroidism such as palpitations, etc.
What antibodies are classically associated with Graves disease and what antibodies are classically associated with Hashimoto thyroiditis?
Graves: TSH receptor antibodies.
Hashimoto: thyroid peroxidase antibodies (TPO) and antithyroglobulin antibodies
What is the nuclear medicine appearance of Graves disease on a thyroid scan?
Regardless of whether imaging is performed with Tc or I123/I131 the thyroid will appear as an
enlarged thyroid gland with homogeneous increased activity. Remember to look for the
pyramidal lobe centrally projecting about the superior aspect of the thyroid gland at midline
above the isthmus. A normal sized thyroid with normal uptake often does not show the
pyramidal lobe. They pyramidal lobe may be seen in something like 10% of normal thyroid
glands and roughly half of all thyroid glands with Graves disease.
What is the classic clinical presentation of a patient with Hashimoto thryroiditis?
A classic clinical history for a patient with Hashimoto thyroiditis would be a middle aged female
presenting with symptoms of hypothyroidism and possible goiter. Note that neck pain is not
classic for Hashimoto thyroiditis. Sometimes Hashimoto thyroiditis can initially present with
hyperthyroidism and subsequent hypothyroidism and, when in the hyperthyroid state, this has
been termed Hashitoxicosis.
What are typical imaging features of Hashimoto thyroiditis on a nuclear medicine thyroid
scan and on an FDG-PET/CT study?
On a nuclear medicine thyroid scan, Hashimoto thyroiditis would show increased uptake in
early stages that can look similar to Graves disease and later would classically appear as an
inhomogeneous thyroid with focal cold areas.
On an FDG-PET/CT study one would expect diffuse uptake throughout the thyroid gland due to
the inflammatory nature of Hashimoto thyroiditis. Any focal hot spots raise suspicion for
possible malignancy.
What malignancy is most classically associated with Hashimoto thyroiditis on board
examinantions?
Primary thyroid lymphoma.
How long must one abstain from breast feeding after receiving Iodine 131?
After receiving I131, lactation must cease for that child (no more breastfeeding). This is due
both to the long half life of I131 (8 days) as well as the risk to the child’s thyroid of ablation and
becoming hypothyroid as a result. I131 is contraindicated in pregnancy and in childhood.
When can breastfeeding resume after receiving Iodine 123 of Tc99m?
I123—about 2-3 days
Tc99m—about 12 hours
Elevated radioiodine uptake can be seen with which classic entities?
Entities associated with increased radioiodine uptake include Graves disease, early Hashimotos,
dietary deficiency of iodine. With dietary deficiency of iodine the thyroid is so hungry for iodine
that when it sees the radioiodine it simply takes up more of it.
How does renal dysfunction affect iodine uptake values?
Renal dysfunction may cause elevated levels of iodine in the blood pool and because the
thyroid has more iodine at baseline it will take up less radiotracer due to competition between
the iodine in the blood pool and the administered radioiodine independent of TSH values. So in
setting of renal dysfunction one would predict lower radioiodine uptake values.
What are some medications that can classically lower radioiodine uptake?
These include thyroid blockers (methimazole, propylthiouracil), nitrates, iodinated contrast via
IV, amiodarone.
What is the classic thyroid scan imaging appearance of toxic multinodular goiter?
Toxic multinodular goiter will show one or more hot nodules on a background of a cold gland as
the hot nodules preferentially take up all or most of the radioactive iodine
What are typical thyroid uptake values and clinical histories for patients with toxic
multinodular goiter?
often only moderately elevated with toxic multinodular goiter, for example uptake around 40% at 24 hours (classic Graves may show uptake around 70% at 24
hours). A classic history would be an elderly female with symptoms of hyperthyroidism such as
tachycardia, weight loss, insomnia, and anxiety.
If a thyroid nodule is cancerous, will this nodule most likely be hot or cold on a radioactive iodiine thyroid scan? What about an FDG-PET/CT scan?
A cancerous thyroid nodule is most likely to be cold on an I123/I131 thyroid scan and hot on an
FDG-PET/CT scan. Remember that normal thyroid takes up radioactive iodine so if a nodule is
cold and therefore does not take up iodine, that means something is wrong with that nodule
compared to normally functioning thyroid tissue—and that can be evidence of cancer with
derangement of normal cellular function. As we all know, cancer can by hypermetabolic and a
focal FDG avid thyroid nodule should raise suspicion for possible malignancy.
Are most cold thyroid nodules cancerous?
No, most cold nodules will be benign. Additionally, multiple cold nodules in the setting of
multinodular goiter are more likely to be benign compared to a single cold nodule.
What is a discordant thyroid nodule?
A discordant thyroid nodule is a thyroid nodule that shows increased uptake on a Tc-99m scan
but decreased or absent uptake on an I131/I123 scan. The problem is that some thyroid cancer
cells may retain enough function to trap but not enough function to organify. Remember that
Tc and radioactive iodine are both trapped but only radioactive iodine is organified. So a
warm/hot nodule on a Tc-99m scan is not necessarily benign as Tc is trapped but not organified
by the thyroid. Take home message is that one needs to be careful calling a nodule benign
based on a Tc-99m thyroid scan only, and a nodule may only be considered benign once you
show the nodule is warm/hot on an iodine thyroid scan.
What are top differential considerations if the thyroid takes up Tc-99m on early images but
not radioactive iodine at 24 hours?
Potential causes include congenital enzyme deficiency that interferes with organification versus
therapy with propylthiouracil that blocks organification.
Do you want TSH to be elevated or low prior to I131 cancer ablation therapy?
TSH should be elevated prior to I131 ablation therapy in order to stimulate residual thyroid
tissue to take up as much I131 as possible. Two primary options exist to raise TSH values. The
first is cessation of thyroid hormone replacement to stimulate the natural TSH release as the
body becomes hypothyroid. The second option is to remain on thyroid hormone and take
recombinant TSH which is also known as thyrogen. Note that a minimum TSH value of 30 is
often considered acceptable pre I131 ablation and a TSH value of 50 or higher may be
considered ideal.
What are common precautions that individuals must take prior to I131 therapy?
Following I131 therapy a patient must isolate for 3 days including bathroom hygiene, sleeping
alone, no exposure of others to bodily fluids including saliva and urine. Additionally, patients
should stay well hydrated.
What is an estimated risk of malignancy from a single cold nodule on a radioactive iodine
scan?
A single cold nodule has an approximate 15 to 20% chance of malignancy. Note that risk of
malignancy increases if a cold nodule is seen in younger patients, if a nodule is hard on
palpation, if there is a history of neck radiation, and with family history of thyroid cancer
What is an estimated risk of malignancy from a hot nodule on a thyroid scan?
In general, the risk of thyroid cancer in a nodule that is hot on a radioactive iodine scan is
thought to be less than 1%.
What should you consider if thyroglobulin levels are rising in a patient who has completed
thyroidectomy and I131 ablation for thyroid cancer and the thyroid I123/I131 scan is
negative? What is the next best test to consider?
One must consider the possibility that the thyroid cancer is truly back, as suggested by the
thyroglobulin levels, but has de-differentiated and therefore no longer takes up radioactive
iodine. In this setting, one would want to perform an FDG-PET/CT study to show where the
site(s) of recurrence are as de-differentiated thyroid cancer tends to be FDG avid.
What type of collimator is best for I131 imaging?
High energy collimators are used for I131 imaging given the very high 364 keV energy. If lower
energy collimators are used you would expect to see septal penetration with the star-like
artifact that is classic for this entity. Note that I131 is basically the highest energy radioisotope
commonly used in general nuclear medicine
Besides thyroid imaging can you name another nuclear medicine scan that uses I123/I131?
MIBG uses I123/I131. Note that one needs to block the thyroid gland before MIBG to prevent
thyroid uptake. Lugols Iodine solution is commonly used prior to imaging to prevent
unnecessary radiation to the thyroid when using MIBG imaging. Remember to block the
thyroid first prior to MIBG imaging
Approximately how long should you wait following IV iodinated contrast administration to
treat a patient with I131?
Typically about 6-8 weeks.
What are NRC guidelines for releasing a patient following I131 oral administration?
- No individual of the public is likely to receive more than 5 mSv exposure from the patient
treated with I131. - When a survey meter reading at 1 meter is less than 0.07 mSv/hour (7 mrem/hour)
- When administered activity is 33 mCi or less.
What is the TSH goal prior to I-131 treatment?
> 30 mIU/L, optimal is 50
What medication can be used if TSH is low prior to I-131 treatment?
Thyrogen
At what thyroglobulin level do you expect recurrence?
> 10 ng/mL
What can you infer if Thyroglobulin levels don’t decrease after re-treatment and RAI scan is negative?
Non-iodine avid tumor
Next step would be FDG/PET
Special precaution: What is the maximum dose of I-131 you can administer to the lungs (in the setting of pulmonary mets)?
<80 mCi of I-131
Special precaution: What is the maximum dose of I-131 you can administer to the bone marrow?
< 200 mGy
