Thyroid Flashcards
What is meant by “trapping” iodine? And what radiotracers do this?
Transported into the gland
I123, I131, and Tc99m all do it
What is meant by “organification”? What radiotracers do this?
Oxidized by thyroid peroxidase and bound to tyrosyl moiety.
I123 and I131 do it - Tc99m does NOT and slowly washes out of the gland
Advantages/Disadvantages of I131? Useful for? Contraindicated in?
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
Advantages/Disadvantages of I-123?
short half life with lower energy (159 keV) via electron capture, so better image but costs more
Why use Tc99m for thyroid?
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)
Breast feeding with I-131, I-123, and Tc99m; when can you resume?
Tc-99m 12-24 hours
I-123 2-3 days
1-131 pump and dump, should not do it
Briefly outline a iodine uptake test. Tracer, amount, normal.
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.
What factors affect the Iodine uptake test? (3)
1) Renal Fxn (increases stable iodine pool, reduces numbers)
2) Dietary Iodine (variable and controversial)
3) Medications (thyroid blockers, nitrates, IV contrast, AMIODARONE
What diseases will have increased uptake on Iodine uptake test? (4)
Graves, Early Hashimoto, Rebound after withdrawal of thyroid medication, dietary iodine deficiency
What disease will have decreased uptake on Iodine uptake test? (4)
Primary or secondary causes of hypothyroid
Renal failure
Medications (thyroid blockers, nitrates, IV contrast, amiodarone)
Diatary Iodine Overload
Diffuse, homogenous uptake in the thyroid of a woman with protracted course, pretibial edema, and exophthalmos?
Graves Disease. Uptake increased at both 4-6 and 24 hours.
Visualization of the pyramidal lobe should be suggestive of what?
Graves disease, only visible 10% of the time normally, 45% of the time in Graves.
Elderly woman with weight loss, anxiety, insomnia, and tachycardia?
Multi-nodular toxic goiter.
Gland uptake pattern for multi-nodular toxic goiter?
Typically heterogeneous, only moderately elevated. Nodules hot on a cold gland background.
How do you differentiate toxic multi-nodular goiter from non-toxic multi-nodular goiter?
Toxic: hot nodules on cold gland
Non-toxic: warm nodules on normal thyroid
Uptake pattern of Grave’s dz?
High: 70s, >50%. Homogeneous.
What is the most common cause of goitrous hypothyroidism in the US?
Hashimotos
Explain Hashimoto (brefly)
AI dz causes hyper -> hypo as gland burns out from stimulation.
During what phase of Hashimoto is it usually detected?
Hypo phase
What malignancy does Hashimoto carry an increased risk of?
Primary thyroid lymphoma
What antibody will be seen in Hashimoto?
antibodies to TPO and anti-thyroglobulin
Appearance of Hashimoto when it is Hypo/Hyper?
Hypo: inhomogeneous with focal cold areas
Hyper: diffusely increased tracer, looks like graves
Viral prodrome -> Hypothyroid?
Subacute (de Quervains) thyroiditis
Thyroid uptake scan for subacute thyroiditis?
DECREASED %RAIU, it can mimic graves in acute phase - Low TSH, High T3, High T4 - check %RAIU
What is a discordant nodule? How do you confirm benign etiology?
Hot on Tc99 but COLD on I123, cancers can maintain ability to trap but not organify -> not benign until hot on I123 too.
Classic: Thyroid takes up Tc but NOT iodine on 24 hr imaging? (2)
Congenital enzyme deficiency, medications that block organification (PTU)
Classic: Have low iodine uptake on 24 hour (no Tc scan to go with it)? (2)
de Quervains or Hashimoto
Which does better with surgery + radiotherapy, Papillary or Medullary, and why?
Papillary - because Medullary doesn’t drink Iodine
What makes you resistant to I131 therapy? (3)
Medullary Suubtype CA, Hx of prior 131 therapy, Hx of Methimazone tx
What radioisotopes can be seen with Med Ca Thyroid? (Hint: neuroendocrine tumor)
Can uptake MIBG or Octreotide - cold on 131 scan
What % thyroid activity is the goal before full ablation?
<5% ideal
What lab value do you want very high for 131 ablation?
TSH
How do you get TSH elevated? (2)
Stop thyroid hormone replacement, or give recombinant TSH “thyrogen”
How do you decide on 131 dosing?
Depends on stage: 100 for thyroid, 150 for thyroid + nodes, 200 for distal dz
When do 131 ablation patients get admitted?
> 7mR/h measured 1m from patient’s chest (some places use 5).
Remember: 33 mCi of residual activity (30 in agreement states)
Side effects of 131 tx? (2)
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.
How does the body get rid of 131?
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.
Is it ok to breast feed after 131? When can a female get pregnant after?
NO - for at least that pregnancy, and no getting pregnant for 6-12 months.
Absolute CI to 131? (2)
Pregnancy and severe uncontrolled thyrotoxicosis
If you see an iodine scan and uptake is in the liver?
Post treatment scan - always
Classic: Dialysis pt needs 131 Rx.
Immediately following dialysis. Decrease dose because no excretion. Dialysate can be dumped in sewer, dialysis tubing stays in storage.
I131 treatment in hyperthyroidism dosing?
Depends on etiology:
Graves - 15 mCi
Multinodular - 30 mCi
Treat thyroid eye disease?
Controversial - some people say it gets worse. If prompted have optho look.
What is the Wolff-Chaikoff Effect?
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)
