Nuclear Medicine and the Thyroid Part 2
Part 2 review of nuclear medicine and the thyroid for radiology board exams.
Show Notes/Study Guide:
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
What are approximate normal values of iodine uptake at 4 and 24 hours?
Range of estimates vary but I remember 5-15% at 4 hours (range can be 4-6 hours) and 10-30% at 24 hours. Note this helps me remember as I simply double the 4 hour 5-15% to remember the 25 hour 10-30% range. On board exam questions they should make it obvious whether a result is in this ballpark range of normal or clearly abnormal. I wouldn’t expect that they would give you an uptake of 3% or 17% at 4 hours and ask you if this is normal.
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 do TSH values help distinguish between Graves disease and dietary iodine deficiency?
Because both of these entities will have increased radioiodine uptake values, one can use TSH to help differentiate these entities (as well as dietary history and clinical history/symptoms). Graves would be expected to have a low TSH value whereas dietary iodine deficiency would be expected to have an elevated TSH level.
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 significance in Graves disease of an elevated 4 hour uptake value and a 24 hour uptake value that is lower than the 4 hour value or even within normal limits?
This denotes rapid thyroid hormone production—so rapid in fact that the tracer was depleted by 24 hours as it already had been trapped and organified and subsequently released from the thyroid. This is a sign of a more active Graves disease that is really ramped up.
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. Hot nodules on a background cold gland is seen with a toxic multinodular goiter whereas warm nodules on a heterogeneous thyroid that demonstrates normal background uptake is seen with multinodular (nontoxic) goiter. For review, Graves disease is expected to show homogeneous diffuse uptake on an enlarged gland and would not show the focal and hetereogeneous uptake seen with toxic and non-toxic multinodular goiter.
What are typical thyroid uptake values and clinical histories for patients with toxic multinodular goiter?
Thyroid uptake values are 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 patient presents with a low TSH and a high T3/T4 what entities should you consider and how can thyroid uptake values help you differentiate between these?
A low TSH and elevated T3/T4 values can be seen with multiple entities including Graves disease, subacute thyroiditis, solitary autonomous hyperfunctioning nodule and toxic multinodular goiter.
Radioiodine uptake would be expected to be low in subacute thyroiditis or dietary iodine overload and elevated in Graves disease, toxic multinodular goiter and solitary autonomous hyperfunctioning nodule.
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.
