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What are 3 Nuclear Medicine Procedures for thyroid and what are 2 pharmaceuticals?
- 1. RAIU: thyroid fx
- 2. Thyroid gland imaging: nodules and cancer
- 3. Thyroid gland treatment: hyperthyroidism and cancer
- A. TcO4
- B. Sodium Iodide (I-123 & I-131)
Describe Negative Feedback System
- -Pituitary Gland constantly senses level of free thyroid hormone
- -If level decreases, pituitary releases TSH which stimulates thyroid.
- -If level increases, pituitary stops releasing TSH and slows production
- *TRH= made in hypothalamus, released into pituitary,
- -aka thyroid topine hormone
- -causes pituitary to produce TSH
Describe how iodine is taken up by the body
- 1. Iodine is eaten or absorbed
- 2. Thyroid has iodide trap (active transport) **TSH speeds up process**
- 3. I changes into I- by oxydation (very reactive)
- 4. I- changes to tyrosine; splits and becomes either MIT or DIT --> whole process is called peroxidase.
Name the 4 biological properties of Radioiodide
- 1. Absorption: complete in 1-2 hrs
- 2. Organ Distribution: thyroid, salivary glands, stomach
- 3. Thyroid blockers: SSKI, xray contrast
- 4. Excretion: Glomerular filtration (73% reabsorbed); urinary is 76% if pts RAUI is normal (15%)
How much SSKI is recommended by the FDA?
1000mg KI or 32mg KI/drop
What are the 3 biological properties of 99mTc O4?
- 1. 2% in gland by 20 min
- 2. Not organified
- 3.Perchlorate will flush any residual
What is the weight of a normal thyroid gland?
- Antibodies stimulate TSH receptors in gland.
- Increased uptake.
- Gland size 2-3x bigger than normal.
- a. Diffuse Goiter (Graves' disease)
- b. Toxic nodular goiter
- -solitary nodule
What are the 2 types of Thyroiditis?
1. Subacute: Viral cause; initially pt is hyperthyroid>hypothyroid>normal
- 2. Chronic: autoimmune disease; thyroid cells replaced by lymphocytes and eventually fibrosis
- hypothryroidism>gland enlarged but decreased uptake of radioiodine
Palpable areas in gland
hot: increase uptake (usually benign)
cold: decreased uptake (6-10% malignant)
Describe Thyroid Uptake study
- 1. Pt is NPO 4hrs and discontinue thyroid hormone and PTU (block organification) for 4 days
- 2. 5-10 mCi I-131 NaI capsule or 100uCi of I-123 NaI
- 3. Count thyroid @ 4 and 24 hr; compare to neck phantom
- Neck CPM-Thigh CPM Dose CPM- BKD CPM X100%
4. NORMAL: 4hrs (5-15%); 24hrs (10-35%)
- *Helpful in determining doese of I131>>Tells you how much will be taken up for hyperthyroidism treatment*
- **I131 better than Tc bc it provides better contrast at 24 hrs**
What is the main role of a thyroid scan?
Determine which nodules should undergo fine needle bx to confirm benignancy
What are the doses and DTST for thyroid imaging pharms?
- DOSE DTST
- I131 50-100uCi (neck) 24hr
- 2 mCi WB 24 hr
- I123 100-200uCi (neck) 24 hr
- 2 mCi WB 24hr
TcO4 10 mCi (neck) 30 min
Describe Graves' Disease (Diffuse Toxic Goiter)
- Enlarged gland; Uniform uptake
- Secretes thyroid hormone 5-15x
- RAIU 50-80%
- Plasma TSH low (high T3&T4>>TSH shuts off)
- Autoimmune disease
- Treatment: Surgery, antithyroid drugs
Describe Plummer's Disease (Toxic Multinodular Goiter)
- Enlarged gland, heterogenous uptake
- Unknown etiology
- Requires 2-3x dose of radioiodine compared to Graves disease
Describe Hashimoto's Thyroiditis
- Autoimmune disease
- Increased gland size
- Increase TSH (LOW T3 & T4)
- Organification defect= high intrathyroidal iodide pool
- May produce abnormal perchlorate washout
- -normal= no washout
- abnormal=RAIU >15% 1 hr post perchlorate admin.
If thyroid cancer appears cold, how does one expect to localize mets?
They appear cold relative to adjacent normal thyroid tissue.
Met lesions are more easily identified bc they reside away from normal thyroid tissue>>>decreased background
*Met usually found in mediastinum, lungs, and bone*
Describe the thyroid cancer protocol
- 1. Thyroidectomy performed
- 2. Patient is made hypothyroid or given rhTSH (thyrogen)3. WB scan @ 24hr after 2 mCi of I131 or I123
- 4. Therapy dose: 150-400 mCi of I131
- 5. Pt released if no one else will be exposed >.5 rem. Hospitalized pts discharged after <33 mCi in body
- 6. Post WB scan 1 yr later
What are the 3 criterias for pt release after I131 therapy?
1. NO public member will receive >.5 rem
2. Survey meter is <7mrem/hr @ 1 meter
3. <33 mCi is left in pt.
Equation for Dosing Method
- Gland Wt(g) x uCi desired/g
- % 24 RAIU x 10
- *Typical doses
- Graves 10mCi
- Plummers 30mCi
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