Thyroid

Question 1

What is meant by “trapping” iodine? And what radiotracers do this?

Answer 1

Transported into the gland

I123, I131, and Tc99m all do it

Question 2

What is meant by “organification”? What radiotracers do this?

Answer 2

Oxidized by thyroid peroxidase and bound to tyrosyl moiety.

I123 and I131 do it - Tc99m does NOT and slowly washes out of the gland

Question 3

Advantages/Disadvantages of I131? Useful for? Contraindicated in?

Answer 3

Advantage: Cheap
Disadvantage: long half life (8 days), high energy (364 keV) beta emitter -> bad images with a 1/2 inch crystal
Useful for therapy, not imaging
CI in kids + pregnant women

Question 4

Advantages/Disadvantages of I-123?

Answer 4

short half life with lower energy (159 keV) via electron capture, so better image but costs more

Question 5

Why use Tc99m for thyroid?

Answer 5

Trapped but not organified so background levels are higher because only 1-5% taken up by thyroid - useful when they’ve had a recent thyroid blocker on (iodinated contrast is sneaky here)

Question 6

Breast feeding with I-131, I-123, and Tc99m; when can you resume?

Answer 6

Tc-99m 12-24 hours
I-123 2-3 days
1-131 pump and dump, should not do it

Question 7

Briefly outline a iodine uptake test. Tracer, amount, normal.

Answer 7

5 mCi I-131 or 10-20 mCi I-123 and report at 4-6 and 24 hours. Normals 5-15% @ 4-6, and 10-25% @ 24 hours. Correction for background done for measurements prior to 24.

Question 8

What factors affect the Iodine uptake test? (3)

Answer 8

1) Renal Fxn (increases stable iodine pool, reduces numbers)
2) Dietary Iodine (variable and controversial)
3) Medications (thyroid blockers, nitrates, IV contrast, AMIODARONE

Question 9

What diseases will have increased uptake on Iodine uptake test? (4)

Answer 9

Graves, Early Hashimoto, Rebound after withdrawal of thyroid medication, dietary iodine deficiency

Question 10

What disease will have decreased uptake on Iodine uptake test? (4)

Answer 10

Primary or secondary causes of hypothyroid
Renal failure
Medications (thyroid blockers, nitrates, IV contrast, amiodarone)
Diatary Iodine Overload

Question 11

Diffuse, homogenous uptake in the thyroid of a woman with protracted course, pretibial edema, and exophthalmos?

Answer 11

Graves Disease. Uptake increased at both 4-6 and 24 hours.

Question 12

Visualization of the pyramidal lobe should be suggestive of what?

Answer 12

Graves disease, only visible 10% of the time normally, 45% of the time in Graves.

Question 13

Elderly woman with weight loss, anxiety, insomnia, and tachycardia?

Answer 13

Multi-nodular toxic goiter.

Question 14

Gland uptake pattern for multi-nodular toxic goiter?

Answer 14

Typically heterogeneous, only moderately elevated. Nodules hot on a cold gland background.

Question 15

How do you differentiate toxic multi-nodular goiter from non-toxic multi-nodular goiter?

Answer 15

Toxic: hot nodules on cold gland

Non-toxic: warm nodules on normal thyroid

Question 16

Uptake pattern of Grave’s dz?

Answer 16

High: 70s, >50%. Homogeneous.

Question 17

What is the most common cause of goitrous hypothyroidism in the US?

Answer 17

Hashimotos

Question 18

Explain Hashimoto (brefly)

Answer 18

AI dz causes hyper -> hypo as gland burns out from stimulation.

Question 19

During what phase of Hashimoto is it usually detected?

Answer 19

Hypo phase

Question 20

What malignancy does Hashimoto carry an increased risk of?

Answer 20

Primary thyroid lymphoma

Question 21

What antibody will be seen in Hashimoto?

Answer 21

antibodies to TPO and anti-thyroglobulin

Question 22

Appearance of Hashimoto when it is Hypo/Hyper?

Answer 22

Hypo: inhomogeneous with focal cold areas
Hyper: diffusely increased tracer, looks like graves

Question 23

Viral prodrome -> Hypothyroid?

Answer 23

Subacute (de Quervains) thyroiditis

Question 24

Thyroid uptake scan for subacute thyroiditis?

Answer 24

DECREASED %RAIU, it can mimic graves in acute phase - Low TSH, High T3, High T4 - check %RAIU

Question 25

What is a discordant nodule? How do you confirm benign etiology?

Answer 25

Hot on Tc99 but COLD on I123, cancers can maintain ability to trap but not organify -> not benign until hot on I123 too.

Question 26

Classic: Thyroid takes up Tc but NOT iodine on 24 hr imaging? (2)

Answer 26

Congenital enzyme deficiency, medications that block organification (PTU)

Question 27

Classic: Have low iodine uptake on 24 hour (no Tc scan to go with it)? (2)

Answer 27

de Quervains or Hashimoto

Question 28

Which does better with surgery + radiotherapy, Papillary or Medullary, and why?

Answer 28

Papillary - because Medullary doesn’t drink Iodine

Question 29

What makes you resistant to I131 therapy? (3)

Answer 29

Medullary Suubtype CA, Hx of prior 131 therapy, Hx of Methimazone tx

Question 30

What radioisotopes can be seen with Med Ca Thyroid? (Hint: neuroendocrine tumor)

Answer 30

Can uptake MIBG or Octreotide - cold on 131 scan

Question 31

What % thyroid activity is the goal before full ablation?

Answer 31

<5% ideal

Question 32

What lab value do you want very high for 131 ablation?

Answer 32

TSH

Question 33

How do you get TSH elevated? (2)

Answer 33

Stop thyroid hormone replacement, or give recombinant TSH “thyrogen”

Question 34

How do you decide on 131 dosing?

Answer 34

Depends on stage: 100 for thyroid, 150 for thyroid + nodes, 200 for distal dz

Question 35

When do 131 ablation patients get admitted?

Answer 35

> 7mR/h measured 1m from patient’s chest (some places use 5).

Remember: 33 mCi of residual activity (30 in agreement states)

Question 36

Side effects of 131 tx? (2)

Answer 36

1) pulm fibrosis if give to pt with lung mets

2) Sjogren have greater risk of salivary gland damage, Salivary gland damage is dose related.

Question 37

How does the body get rid of 131?

Answer 37

Urine, also sweat, tears, saliva, breast milk - home precautions based on that. Ex. no sex/kissing/contact x3 days, guys sit down to pee etc. etc. etc.

Question 38

Is it ok to breast feed after 131? When can a female get pregnant after?

Answer 38

NO - for at least that pregnancy, and no getting pregnant for 6-12 months.

Question 39

Absolute CI to 131? (2)

Answer 39

Pregnancy and severe uncontrolled thyrotoxicosis

Question 40

If you see an iodine scan and uptake is in the liver?

Answer 40

Post treatment scan - always

Question 41

Classic: Dialysis pt needs 131 Rx.

Answer 41

Immediately following dialysis. Decrease dose because no excretion. Dialysate can be dumped in sewer, dialysis tubing stays in storage.

Question 42

I131 treatment in hyperthyroidism dosing?

Answer 42

Depends on etiology:
Graves - 15 mCi
Multinodular - 30 mCi

Question 43

Treat thyroid eye disease?

Answer 43

Controversial - some people say it gets worse. If prompted have optho look.

Question 44

What is the Wolff-Chaikoff Effect?

Answer 44

Reduction in thyroid hormone levels caused by ingestion of a large amount of iodine lasting several days followed by escape phenomenon. Used as a principle to tx thyroid storm. Also explains why hypothyroidism sometimes happens with several iodine containing drugs (amiodarone)