Thyroid Flashcards
radionuclides transported in the thyroid gland
I-123, I-131, Tc-99m
Which radionuclide(s) are transported into the thyroid but not organified?
Tc-99m
What is special about Tc-99m in the thyroid
transported but not organified
I-131 energy
365 keV
I-131 half life
8 days
8 day half life
I-131
365 keV
I-131
13 hour half life
I-123
I-123 half life
13 hours
159 keV
I-123
I-123 energy
159 keV
Pt with recent iodinated contrast and Jod-Basedow phenomenon comes in for imaging of the thyroid. What imaging agent will you give them?
Tc-99m-O4. (pertechnetate)
TYPICALLY you would choose I-123 except in scenarios where the thyroid won’t take up iodine normally –> aka thyroid blocker or recent iodinated contrast
you can resume breast feeding in 24hr with what thyroid imaging radiotracer?
Tc-99m
you can resume breast feeding in 48hr with what thyroid imaging radiotracer?
I-123
you can NEVER resume breast feeding with what thyroid radionuclide?
I-131
Breast feeding trivia for :
Tc99m, I-123, I-131
Tc99m: resume in 24 hr
I-123: resume in 48 hour
I-131: NEVER for the current baby
How much I-131 or I-123 is given for iodine uptake tests?
5 mCi of I-131
or
10-20 mCi of I-123
What are the time points at which thyroid uptake is measured on an iodine uptake test?
4-6 hours AND 24 hrs
Normal % uptakes in iodine uptake test
5-15% (4-6 hr)
10-30% (24 hr)
the 4 hr is doubled to remember the 24 hr range
medications that decrease thyroid uptake
thyroid blockers, nitrates, IV contrast, AMIODARONE
How does kidney function affect thyroid uptake
reduced kidney function –> increases stable iodine pool (decreased excretion) –> decreased uptake
high T3 and T4, low TSH, diffuse thyroid uptake
Graves disease
Situations in which thyroid will take up Tc but NOT Iodine on 24 hr imaging
- congenital enzyme deficiency that inhibits organification
- drug like PTU that blocks organification
- Jod-Basedow phenomenon/recent IV contrast administration
Most common subtype of thyroid cancer
papillary
Factors that make something in the thyroid resistant to treatment with I-131
- Medullary Subtype CA (does not drink the tracer since it is neuroendocrine origin)
- History of prior I-131 (“easy gland has been killed off”) –> have to use about 50% more during re-treatment than the original dose
- History of Methimazole treatment (Even if years ago)
Medullary subtype CA association
MEN 2
medullary subtype is neuroendocrine in origin
Thyroid cancer that takes up MIBG or octreotide
Medullary subtype (neuroendocrine in origin)
thyroid cancer that is neuroendocrine in origin
Medullary subtype CA