Case 1 - no activity on Pertechnetate scan - most likely subacute thyroiditis
Case 2 - Single nodule with intense uptake and suppression of the rest of thyroid - autonomous nodule
Case 5- Cold Nodule - concern for thyroid carcinoma which is what it turned out to be.
- I-123 and Technitium-99m pertechnetate are most commonly used imaging radionuclides
- I-131 can also be used for uptake
- I-131 - Beta emitter and has 1/2 life of 8 days. principal gamma emission is 364 emitter. I-131 advantage - low price and ready availability. Disadvantage - long 1/2 life and high-beta emission (esp. to thyroid - about 1 rad/microCi); good for tumor follow up due to long half life
- I-123 - same behavior as iodine. Nuclide of choice for thyroid imaging. Decays by electron capture - 159 KeV and 1/2 of 13 hours. Low background activity. Disadvantages - high cost (produced by cyclotron), availability and delivery.
- Tech 99m - trapped by thyroid but not organified; 6 hour 1/2 life; 140 KeV - ideal for gamma camera; can give higher doses and acquire quickly w/ less risk of motion; higher background than iodine (only 1-5% trapped); Salivary glands - well seen; preferred nuclide when patient has been on thyroid blocking agents or unable to take medication orally or study must be completed in less than 2 hours.
- Both Tech and iodine - cross placenta and secreted in breast milk
- NRC recommends stopping breast feeding if I-131 exceeds 1 microCi
- I-131 - not recommended for children.
- Radioiodine Uptake test - Useful for clinical index of thyroid function.
- Often for determining how much uptake will be present for therapy.
- Useful for differentiating Grave's from subacute thyroiditis or factitious hyperthyroidism
- Basic idea - the more active the gland the more iodine will be taken up. Measurements obtained at 4-6 hours and 24 hours post administration.
- 4-6 - normal is 6-18%; 24 - 10-30%
- Patient should be NPO after midnight
- I-123 10-20 microCi; I-131 5 microCi
- Neck compared with neck phantom. Probe - 25-30 cm from anterior neck
- Equation - (neck counts-thigh counts)/counts in standard1
- 4-6 hour uptake recommended - esp for the possibility of a rapid turnover Grave's
- Factors affecting uptake - Too much iodine in diet (low uptake); too little (high uptake)
- Increased uptake factors - hyperthyroidism, early hashimoto's, recovery from subacute thyroiditis, Rebound after antithyroid meds; Enzyme defects; Iodine deficiency; Hypoalbuminemia; TSH; tumor secreted stimulators; Pregnancy
- Decreased uptake factors - hypothyroidism; iodine overload (contrast); meds; subacute or autoimmune thyroiditis; thyroid hormone tx; ectopic secretion from tumors (struma-ovarii etc); renal failure
- Compounds that decrease uptake - adrenocorticosteroids, amiodarone, bromides, butazolidine, Mercurials, methimazole, Propylthiouracil; nitrates; Perchlorate; salicylates; kelp, Cytomel, synthroid, contrast
- Renal failure - iodides are retained leading to larger pool of iodine which will compete with I-123 or I-1316.
- B-blockers - do not affect function of thyroid and does not affect uptake.
- Chronic Thyroiditis - Hashimoto's - most common form of inflammatory disease of thyroid
- Likely Autoimmune; usually female
- Thyromegaly - presenting w/ possibly mild hyperthyroidism or hypothyroidism
- Early - scan shows diffusely uniform like Grave's
- Later - coarse patchy activity - similar to multinodular goiter
- Acute (bacterial), subacute (viral or autoimmune) - usually diagnosed on clinical or physical grounds - scanning little role
- Subacute - painful swollen gland w/ elevated thyroid hormone levels but depressed uptake - little or no localization
- Cold Nodule - Most common - colloid cyst (70-75%)
- Carcinoma - 15-25% - this is why every cold nodule should get US and possible biopsy.
- Miscellaneous - <15%>
- Factors increasing risk of malignancy - younger patients, male, hx of rads to head and neck, hard lesion, other masses in neck, no shrinkage on thyroid hormone, family hx.
- Factors decreasing risk - older, female, sudden onset, tender or soft lesion, multiple nodules, shrinkage on thyroid hormone.
- US - useful for showing solid versus cystic (not benign vs malignant).
- Hot nodules almost always represent hyperfunctioning adenomas of which 1/2 are autonomous
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- Often not suppressible by thyroid hormone and independent of pituitary axis.
- Can suppress surrounding thyroid
- Warm - may represent a cold nodule that has overlying normal thyroid tissue - so have to get multiple views.
