Thyroid

Question 1

Thyroid : I 131

Answer 1

beta and 364 keV gamma photons
half life: 8 days
production: generator
use: therapy (thyroid cancer s/p thyroidectomy), hyperthyroidism from Graves, multinodular gland

Question 2

Thyroid: I 123

Answer 2

decay: electron capture; produces 158 keV gamma photons
half life: 13 hours
production: cyclotron ($$$)
use: thyroid imaging

Question 3

Thyroid: Tc 99 pertechnetate

Answer 3

decay: 140 keV gamma photon
half life: 6 hours
use: after initial uptake goes in blood pool, not thyroid specific; but can also see salivary glands/thyroid; high background counts

administration: IV

Question 4

Tc 99 pertecnetate vs I-123

Answer 4

Tc99 pertechnetate preferred over I-123 when pt received recent IV contrast, when IV meds are necessary, or when quick study is performed

Question 5

radiotracers and pregnancy/breast feeding: I-131, I-123, Tc 99m

Answer 5

I-131 contraindicated in pregnancy as can cross the placenta; fetal iodine takes up at 12 weeks gestation

I-131 must stop breast feeding if under ablative dose

I-123 can resume breast feeding 2-3 days

Tc99m can resume breast feeding 12-24 hrs after administration

Question 6

preparing for I-123/I-131 therapy

Answer 6

must have nonsuppressed TSH so stop taking exogenous thyroid hormone for 4 weeks or 2 weeks if IM injections of recombinant TSH

Question 7

thyroid indications for I-123 or Tc pertecnetate

Answer 7

ectopic thyroid, thyroid nodule, Graves disease, Hashimoto thyroiditis, subacute thyroiditis

Question 8

ectopic thyroid tissue, diagnosis

Answer 8

I-123 or Tc99m can diagnose lingual thyroid, thyroid, thyroid in ovarian teratoma (struma ovarii), retrosternal thyroid tissue (substernal goiter)

Question 9

thyroid nodule: diagnosis

Answer 9

imaged if cytology indeterminant

hot: retains uptake; usually hyperfunctioning benign adenoma; biopsy if discordant (Tc99m hot, I-123 cold)

cold nodules: 20% malignancy, although usually benign
colloid cyst

Question 10

diagnosis Graves disease

Answer 10

Graves: hyperthyroid, thyromegaly, increased thyroid activity

uptake elevated at 6 and 24 hours

treat with I-131 therapy or surgery; methimaozle/or PTU may also be used after 1-2 yrs of treatment

Question 11

diagnosis: hashimoto thyroiditis

Answer 11

thyromegaly with variable thyroid levels (usually hypothyroid)

patchy appearance of scan

Question 12

diagnosis: subacute thyroiditis

Answer 12

swollen, painful gland; silent hyperthyroidism

decreased radiotracer uptake, low 24 hr uptake

treatment self limited with NSAIDS or steroids

Question 13

therapeutic I-131 indications

Answer 13

s/p thyroid carcinoma thyroidectomy, post radioidine ablation, treat graves disease, multinodular goiter

Question 14

I-131 thyroid carcinoma post thyroidectomy

Answer 14

1-2 mo after thyroidectomy, use to image/ablate residual thyroid tissue

withhold exogenous TSH so TSH 30-50 mmu/mL

2 doses

• low dose <30 mCi for small tumors, no invasion, low-risk
• high dose/high risk: 100-200 mCi I-131

newer approach 30 mCi for treatment of all T1, T2 and N1 cancers

Question 15

therapeutic I-131 for thyroid carcinoma, post radioidine therapy

Answer 15

after ablation with I-131, monitor thyroglobulin levels, if they rise, rescan with I-123

if I-123 is positive, repeat I-131 for ablation

Question 16

therapeutic I-131 for Graves disease

Answer 16

single oral dose of I-131, usually based on estimated thyroid weight, 24 hr uptake

contraindications are pregnancy, lactation, and inability to comply with radiation safety guidelines