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Core > Nuclear: Thyroid > Flashcards

Nuclear: Thyroid Flashcards

1
Q

What is the difference between thyroid radiotracer “trapping” and “organification”

A

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)

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2
Q

When does fetal thyroid formation occur?

A

8-12 weeks gestation

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3
Q

Which is better for imaging: I-131 or I-123?

A

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.

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4
Q

Why might you chose Tc-99m for a thyroid scan over I-123/I-131?

A

If the pt has recently received a thyroid blocker (such as iodinated contrast on a recent CT)

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5
Q

When can you resume breastfeeding after having a nuclear thyroid scan?

A

I-123: 2-3 days later
I-131: No more breast feeding until next pregnancy
Tc-99m: 12-24 hours later

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6
Q

How is an iodine uptake test performed

A

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.

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7
Q

What is a normal iodine uptake result?

A

5-15% (at 4-6 hours)

10-35% (at 24 hours)

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8
Q

What confounding factors affect an iodine uptake test?

A
  • Renal function
  • Dietary iodine
  • Medications (thyroid blockers, nitrates, IV contrast, amiodarone)
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9
Q

Differential for increased iodine uptake on iodine uptake test

A
  • Graves disease
  • Early Hashimotos
  • Rebound after abrupt withdrawal of antithyroid medication
  • Dietary iodine deficiency
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10
Q

Differential for decreased iodine uptake on an iodine uptake test

A
  • Hypothyroidism (primary and secondary causes)
  • Renal failure
  • Medications (thyroid blockers, nitrates, IV contrast, amiodarone)
  • Dietary iodine overload
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11
Q

Why does renal failure cause decreased iodine uptake on iodine uptake tests?

A

Renal failure increases the stable radiotracer pool in soft tissues, which reduces uptake numbers (which are relative to background)

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12
Q

Which medications can decrease iodine uptake on an iodine uptake test?

A
  • Thyroid blockers
  • Nitrates
  • IV contrast (iodinated)
  • Amiodarone
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13
Q

Uptake at 5 hours: 70%
Uptake at 24 hours: 40%

A

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.

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14
Q
A

Pyramidal lobe uptake, suggestive of Graves disease

Note: The pyramidal lobe is seen in 10% of normal pts, but in 45% of Graves pts.

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15
Q

Elderly female with weight loss, anxiety, and tachycardia

A

Think toxic multinodular goiter (AKA Plummer disease)

Note: Heterogeneous thyroid uptake (Graves would be homogenous, see lower image).

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16
Q

Tc-99m thyroid scintigraphy

A

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).

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17
Q

Hashimoto’s increases the risk of what type of cancer?

A

Primary thyroid lymphoma

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18
Q

Thyroid scintigraphy

A

Subacute thyroiditis (AKA De Quervains/granulomatous thyroiditis)

Note: Decreased iodine uptake. This can also be seen in burned out Hashimoto’s.

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19
Q

Upper respiratory infection followed by transient hyperthyroidism…

A

Subacute thyroiditis (AKA De Quervains/granulomatous thyroiditis)

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20
Q

Radioactive iodine uptake at 5 hours is 4%…

A

Subacute thyroiditis (AKA De Quervains/granulomatous thyroiditis)

Note: This can also be seen in burned out Hashimoto’s.

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21
Q

Which is more suspicious for cancer?

A

Cold nodule

Note: Cold nodules are still only cancer in 20-40% of cases, but hot nodules are cancer <1% of the time.

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22
Q

Most likely diagnosis

A

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.

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23
Q

Tc-99m thyroid scintigraphy

A

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).

24
Q

Discordant thyroid nodule

A

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.

25
Q

Thyroid uptake on Tc-99m scan, but no thyroid uptake on 24 hour I-123 scan…

A

Problem with organification (e.g. congenital enzyme deficiency, thyroid blockers like propylthiouracil, etc.)

26
Q

What is the most common subtype of thyroid cancer?

A

Papillay

Note: Papillary is popular.

27
Q

What factors make I-131 treatment of thyroid cancer less successful?

A
  • Medullary subtype
  • History of prior I-131 treatment (easy gland has been killed off)
  • History of prior methimazole treatment (even if years ago)
28
Q

What is the ideal iodine uptake after surgical resection of thyroid cancer prior to treatment with I-131?

A

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.

29
Q

What should serum TSH be prior to I-131 ablation of thyroid cancer?

A

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.

30
Q

What subtype of thyroid cancer can sometimes (10%) show up on MIBG or octreotide scans?

A

Medullary

Note: This is because medullary thyroid cancer is neuroendocrine in origin.

31
Q

What is the best whole body scan for papillary thyroid cancer?

A

I-123 or I-131

32
Q

What is the best whole body scan for medullary thyroid cancer?

A
  • 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.

33
Q

Can PET/CT be used for medullary thyroid cancer?

A

Yes, but only if it is aggressive (only sensitive if calcitonin is at least > 500 and ideally > 1000)

34
Q

How can you get serum TSH up prior to I-131 ablation?

A
  • Stop levothyroxine
  • Give recombinant TSH (i.e. Thyrogen)
35
Q

What is the I-131 dose for thyroid cancer?

A

Depends on stage of disease:

Thyroid only: 100 mCi
Thyroid + nodes: 150 mCi
Thyroid + distant mets: 200 mCi

36
Q

Can a pt go home after receiving I-131 ablation?

A

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.

37
Q

Complications of I-131 ablation

A
  • 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.

38
Q

What are the routes of elimination of I-131 from the body?

A
  • Urine (main)
  • Sweat
  • Saliva
  • Tears
  • Breast milk
39
Q

What precautions do I-131 treatment pts need to follow at home?

A
  • 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.

40
Q

How long should you wait to get pregnant following I-131 treatment?

A

6-12 months

41
Q

How long should you wait after I-131 treatment to resume breast feeding?

A

No more breast feeding for current child

Note: If the pt gets pregnant again (6-12 months later), they can breastfeed.

42
Q

What must be done after discharging a pt who was treated with I-131?

A

The radiation safety officer needs to inspect the pts hospital room before the janitor can clean it and the next pt can move in

43
Q

What thyroglobulin level is concerning following I-131 ablation?

A

Anything over 0 is concerning for recurrence

Note: The trend is more important than the actual number.

44
Q

What lab work is used to monitor for recurrence following I-131 ablation?

A

Thyroglobulin

Note: Anything over 0 is concerning for recurrence.

45
Q

I-131 scan

A

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.

46
Q

I-123 scan showing uptake in the liver…

A

Liver uptake indicates that it is a post I-131 treatment scan

47
Q

Absolute contraindications to I-131 treatment

A
  • Severe uncontrolled thyrotoxicosis (due to risk of thyroid storm)
  • Pregnancy
48
Q

How should you perform I-131 ablation on a dialysis pt?

A
  • 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)
49
Q

What should you do with the dialysate of a dialysis pt who was recently treated with I-131?

A

The dialysate can go down the sewer

Note: The dialysis tubing needs to stay in storage.

50
Q

What I-131 dose should be used to treat hyperthyroidism?

A

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.

51
Q

When should symptoms get better after I-131 treatment of hyperthyroidism?

A

3-4 months later

Note: If there is no clinical evidence of resolution by this point, treatment was likely unsuccessful.

52
Q

Treatment for severe thyrotoxicosis

A
  • Methimazole
  • Propylthiouracil (if first trimester pregnancy or neutropenic pt)

Note: Do not give I-131 (absolute contraindication).

53
Q

Does I-131 improve thyroid eye disease?

A

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.

54
Q

Wolff-Chaikoff effect

A

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”)

55
Q

Why is it possible to treat thyroid storm by infusing large amounts of iodine?

A

The Wollf-Chaikoff effect (large iodine bolus causes a reduction in thyroid hormone levels for approximately 10 days)

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