Nuclear Medicine and the Thyroid Part 1
Q&A review of nuclear medicine and the thyroid for radiology board exams.
Show Notes/Study Guide:
What are the energies and half-lives of Iodine123 and Iodine131?
I131: 364 keV and 8 day half-life. Remember this is the beta emitter and therapeutic agent.
I123: 159 keV and 13 hour half-life. This provides overall superior image quality but cannot be used for therapy and is typically more expensive than I131.
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.
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. I123/I131 will get trapped and then organified allowing the iodine to remain in the gland for a long time, thus facilitating delayed imaging of the thyroid.
Also, although Tc is not iodine, Tc is essentially trapped by the thyroid but is not organified so Tc will wash out of the thyroid much faster than I123/I131.
Tc-99m is trapped but not organified and will show higher background counts as most of the Tc does not end up in the thyroid.
I123/I131 are both trapped and organified and has less background uptake as a higher percentage of the iodine will end up in the thyroid gland vs other tissues.
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.
What are the classic radioiodine uptake values (high, normal, low) for De Quervain thyroiditis?
Low radioiodine uptake in a hyperthyroid patient should make you think of De Quervain thyroiditis, especially in the appropriate clinical setting.
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 examiantions?
Primary thyroid lymphoma.
