Nuclear: Thyroid Flashcards
What is the difference between thyroid radiotracer “trapping” and “organification”
Radiotracer trapping occurs when the radiotracer gets transported into the thyroid gland
Radiotracer organification occurs when the radiotracer is oxidized, incorporated into thyroid hormone, and stored in the thyroid (only I-123 and I-131 do this, Tc-99m does not undergo organification but instead slowly washes out of the thyroid)
When does fetal thyroid formation occur?
8-12 weeks gestation
Which is better for imaging: I-131 or I-123?
I-123 (shorter half life and better energy)
Note: I-131 is sometimes used because its cheaper. Tc-99m produces even worse images than I-131.
Why might you chose Tc-99m for a thyroid scan over I-123/I-131?
If the pt has recently received a thyroid blocker (such as iodinated contrast on a recent CT)
When can you resume breastfeeding after having a nuclear thyroid scan?
I-123: 2-3 days later
I-131: No more breast feeding until next pregnancy
Tc-99m: 12-24 hours later
How is an iodine uptake test performed
Give the pt either 5 micro-Ci or I-131 or 10-20 micro-Ci of I-123 and measure thyroid uptake at 4-6 hours and at 24 hours
Note: You correct for background using neck-thigh counts.
What is a normal iodine uptake result?
5-15% (at 4-6 hours)
10-35% (at 24 hours)
What confounding factors affect an iodine uptake test?
- Renal function
- Dietary iodine
- Medications (thyroid blockers, nitrates, IV contrast, amiodarone)
Differential for increased iodine uptake on iodine uptake test
- Graves disease
- Early Hashimotos
- Rebound after abrupt withdrawal of antithyroid medication
- Dietary iodine deficiency
Differential for decreased iodine uptake on an iodine uptake test
- Hypothyroidism (primary and secondary causes)
- Renal failure
- Medications (thyroid blockers, nitrates, IV contrast, amiodarone)
- Dietary iodine overload
Why does renal failure cause decreased iodine uptake on iodine uptake tests?
Renal failure increases the stable radiotracer pool in soft tissues, which reduces uptake numbers (which are relative to background)
Which medications can decrease iodine uptake on an iodine uptake test?
- Thyroid blockers
- Nitrates
- IV contrast (iodinated)
- Amiodarone
Uptake at 5 hours: 70%
Uptake at 24 hours: 40%
Graves disease (early Hashimoto’s can also look like this)
Normal uptake: 5-15% (at 4-6 hours) and 10-35% (at 24 hours)
Note: Uptake > 50%, think Graves. Uptake may be lower at 24 hours than at 4-6 hours in Graves due to rapid thyroid hormone production.
Pyramidal lobe uptake, suggestive of Graves disease
Note: The pyramidal lobe is seen in 10% of normal pts, but in 45% of Graves pts.
Elderly female with weight loss, anxiety, and tachycardia
Think toxic multinodular goiter (AKA Plummer disease)
Note: Heterogeneous thyroid uptake (Graves would be homogenous, see lower image).
Tc-99m thyroid scintigraphy
Think nontoxic multinodular thyroid
Note: Mildly hot nodules on a background of normal thyroid uptake. Toxic goiter usually results in hot nodules against a photopenic background (picture).
Hashimoto’s increases the risk of what type of cancer?
Primary thyroid lymphoma
Thyroid scintigraphy
Subacute thyroiditis (AKA De Quervains/granulomatous thyroiditis)
Note: Decreased iodine uptake. This can also be seen in burned out Hashimoto’s.
Upper respiratory infection followed by transient hyperthyroidism…
Subacute thyroiditis (AKA De Quervains/granulomatous thyroiditis)
Radioactive iodine uptake at 5 hours is 4%…
Subacute thyroiditis (AKA De Quervains/granulomatous thyroiditis)
Note: This can also be seen in burned out Hashimoto’s.
Which is more suspicious for cancer?
Cold nodule
Note: Cold nodules are still only cancer in 20-40% of cases, but hot nodules are cancer <1% of the time.
Most likely diagnosis
Benign nodule (e.g. colloid cyst, simple cyst, etc.)
Note: Cold nodules are suspicious for cancer (20-40%), but are still more likely to be benign.
Tc-99m thyroid scintigraphy
Thyroid nodule (you shouldn’t consider it benign unless you show that it is also hot on I-123/I-131)
Note: Some cancers can still trap Tc-99m, but can’t organify I-123/I-131 (would be cold on those scans).
Discordant thyroid nodule
A nodule that is hot on Tc-99m scan but cold on I-123 scan
Note: This is concerning for cancer (just like any other I-123 cold nodules). The take-home point is to not rely on Tc-99m scans to determine whether a nodule is functional (benign) or not.
Thyroid uptake on Tc-99m scan, but no thyroid uptake on 24 hour I-123 scan…
Problem with organification (e.g. congenital enzyme deficiency, thyroid blockers like propylthiouracil, etc.)
What is the most common subtype of thyroid cancer?
Papillay
Note: Papillary is popular.
What factors make I-131 treatment of thyroid cancer less successful?
- Medullary subtype
- History of prior I-131 treatment (easy gland has been killed off)
- History of prior methimazole treatment (even if years ago)
What is the ideal iodine uptake after surgical resection of thyroid cancer prior to treatment with I-131?
Less than 5%
Note: If there is more than 5% I-131 uptake on the initial scan following surgical resection, there is a lot of residual thyroid tissue and the I-131 ablation will likely be painful (may need steroids on top of NSAIDs) and you may need to go back to the OR.
What should serum TSH be prior to I-131 ablation of thyroid cancer?
Greater than 30 (ideally 50)
Note: The higher the TSH, the “thirstier” the residual thyroid tissue, and the more it will take up I-131.
What subtype of thyroid cancer can sometimes (10%) show up on MIBG or octreotide scans?
Medullary
Note: This is because medullary thyroid cancer is neuroendocrine in origin.
What is the best whole body scan for papillary thyroid cancer?
I-123 or I-131
What is the best whole body scan for medullary thyroid cancer?
- Octreotide scan (better than MIBG)
- PET/CT (only if calcitonin is over 500)
Note: Medullary thyroid cancer is neuroendocrine and does not take up I-123/I-131.
Can PET/CT be used for medullary thyroid cancer?
Yes, but only if it is aggressive (only sensitive if calcitonin is at least > 500 and ideally > 1000)
How can you get serum TSH up prior to I-131 ablation?
- Stop levothyroxine
- Give recombinant TSH (i.e. Thyrogen)
What is the I-131 dose for thyroid cancer?
Depends on stage of disease:
Thyroid only: 100 mCi
Thyroid + nodes: 150 mCi
Thyroid + distant mets: 200 mCi
Can a pt go home after receiving I-131 ablation?
Yes, as long as there is less than 5 mR/h measured at 1 meter from the pts chest
Note: Pts receiving 150 mCi or less are fine. If higher, they may need to be treated as an inpatient.
Complications of I-131 ablation
- Pulmonary fibrosis (if there are diffuse macronodular lung metastases; micro nodular disease is usually fine)
- Salivary gland damage (higher risk in Sjogrens pts)
Note: Hard candies can help reduce salivary gland dysfunction by increasing saliva production.
What are the routes of elimination of I-131 from the body?
- Urine (main)
- Sweat
- Saliva
- Tears
- Breast milk
What precautions do I-131 treatment pts need to follow at home?
- Drink lots of water
- Suck on hard candies
- Distance from others as much as possible (sleep alone for at least 3 days)
- Bathroom decontamination (sit down to urinate and flush twice)
- Use disposable utensils and plates
- Clothes and linens should be washed separately
Note: Most precautions are done for 3 days following treatment.
How long should you wait to get pregnant following I-131 treatment?
6-12 months
How long should you wait after I-131 treatment to resume breast feeding?
No more breast feeding for current child
Note: If the pt gets pregnant again (6-12 months later), they can breastfeed.
What must be done after discharging a pt who was treated with I-131?
The radiation safety officer needs to inspect the pts hospital room before the janitor can clean it and the next pt can move in
What thyroglobulin level is concerning following I-131 ablation?
Anything over 0 is concerning for recurrence
Note: The trend is more important than the actual number.
What lab work is used to monitor for recurrence following I-131 ablation?
Thyroglobulin
Note: Anything over 0 is concerning for recurrence.
I-131 scan
Radiotracer uptake in thyroid remnant in the thyroid bed
Note: You know this is a post-treatment I-131 scan because there is uptake in the liver.
I-123 scan showing uptake in the liver…
Liver uptake indicates that it is a post I-131 treatment scan
Absolute contraindications to I-131 treatment
- Severe uncontrolled thyrotoxicosis (due to risk of thyroid storm)
- Pregnancy
How should you perform I-131 ablation on a dialysis pt?
- Give dose immediately after a dialysis session (to maximize radiation time)
- Decrease I-131 dose (there is essentially no excretion until the next dialysis session)
What should you do with the dialysate of a dialysis pt who was recently treated with I-131?
The dialysate can go down the sewer
Note: The dialysis tubing needs to stay in storage.
What I-131 dose should be used to treat hyperthyroidism?
Graves disease: 15 mCi (more vascular)
Toxic multinodular goiter: 30 mCi (harder to treat due to capsule)
Note: Just like with thyroid cancer, make sure TSH is high enough before giving dose.
When should symptoms get better after I-131 treatment of hyperthyroidism?
3-4 months later
Note: If there is no clinical evidence of resolution by this point, treatment was likely unsuccessful.
Treatment for severe thyrotoxicosis
- Methimazole
- Propylthiouracil (if first trimester pregnancy or neutropenic pt)
Note: Do not give I-131 (absolute contraindication).
Does I-131 improve thyroid eye disease?
No, in fact it might make thyroid eye disease worse
Note: If the pt has severe thyroid eye disease, you should get an ophthalmology consult prior to treatment with I-131.
Wolff-Chaikoff effect
A reduction in serum thyroid hormone levels cause by the ingestion of a large amount of iodine (this lasts approximately 10 days after ingestion and is followed by an “escape phenomenon”)
Why is it possible to treat thyroid storm by infusing large amounts of iodine?
The Wollf-Chaikoff effect (large iodine bolus causes a reduction in thyroid hormone levels for approximately 10 days)
