Thyroid Anatomy/Physiology/Thyroid scintigraphy principles
NMT631
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Thyroid - position
• The thyroid is a butterfly shaped gland that sits in front of the trachea, midway between the thyroid cartilage (“Adam’s apple”) and the top of the sternum
Supra Sternal Notch
SSN
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Thyroid - Anatomy
A. Normal Adult Thyroid weighs 20-25g
B. Components
1) Two Lateral lobes 2) Isthmus centrally connects the
lobes
3) A Pyramidal lobe in 50% of the population
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Thyroid – Cellular Anatomy
• Epithelial cells arranged in spheres called thyroid follicles.
• Follicles are filled with colloid - Contains two active hormones, T3 and T4- bound to thyroglobulin
• Parafollicular or C cells secrete hypercalcemia hormone calcitonin.
Microscopic structure of Thyroid follicles
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Thyroid Hormone Synthesis1) Absorption
2) Trapping
3) Organification
4) Coupling
5) Release
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Absorption
• Typical diet contains 300 -1000 μg of Iodine/day
• Iodine ingested in food is reduced to iodide, then absorbed into the blood
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Trapping – Iodide pump/trap
• Iodine Pump:
30-50% of the circulating iodine taken up (rest lost in urine)
“Iodine trapping” achieved by active transport (iodide pump).
Iodide pump works against both electrical and concentration gradients.
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Trapping
• Iodide then trapped by follicular cells
• Trapped iodide ions are oxidized by peroxidase enzyme.
IP – Iodide Pump
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Organification – Thyroglobulin’s (TG) role
• TG is a large protein in the colloid
• It has 140 molecules of aminoacid tyrosin used for thyroid hormone synthesis
• Synthesis takes place within the TG molecules in the colloid.
• The trapped iodine is now capable of combining with tyrosine
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Organification• Iodine is linked to tyrosine (an amino acid) on thyroglobulin molecule
• Iodine + tyrosine MIT & DIT
MIT = monoiodotyrosine DIT = diiodotyrosine
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Coupling• With the help of an enzyme, MIT and DIT combine to
form T3 and T4 (thyroxine)
MIT + DIT T3
DIT + DIT T4• Both hormones are stored in thyroid follicular cells and bound to thyroglobulin
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Release• In response to TSH (thyroid stimulating hormone),
T3 and T4 split from thyroglobulin to be sent into the circulation
Steps:
1) TG taken up by thyroid cells by pinocytosis
2) Lysosomes digest thyroglobluin, thereby liberating T3, T4, MIT & DIT.
3) Fee thyroid hormones (T3 & T4) diffuse out through the cell membrane into blood
4) In the circulation, T3 and T4 are bound to TBG (thyroxine-binding globulin)
5) MIT & DIT are deiodinated by an enzyme and is recycled.
13 TSH stimulates iodide trap, thyroid hormone synthesis & release
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Thyroid Hormone synthesis - Summary
• A normal thyroid produces about 10x more T4 than T3
Concise Human Physiology M. Y. Sukkar, H. a. El-Munshid, M. S. M. Ardawi
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Thyroid hormone regulation
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Thyroid Hormone Regulation
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Hypothalamus – Pituitary – Thyroid feedback system
• The activity of the thyroid gland regulated by neuroendocrine negative feedback loop
• maintains a stable amount
of thyroid hormones in the circulation.
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THYROID IMAGING
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Thyroid Imaging
Clinical Indications
Radiopharmaceutical used
Dosage & Administration
Technique
The Normal Scan
Artifacts & Pitfalls
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1) Relate structure to function
2) Evaluation of thyroid nodules
3) Location of ectopic tissue
4) Follow-up exams (post therapy, post surgery)
Clinical Indications
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1) Tc-99m pertechnetate
2) I-123 sodium iodide
3) (I-131 sodium iodide)
Radiopharmaceuticals used
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1) Tc-99m Pertechnetate (99mTcO4-)
• Can substitute for iodide in the iodide transport channel in follicular cells
• Trapped but not organified• Released from cells over time
Radiopharmaceuticals used
• Only 1– 5% of injected dose trapped (↑ Bkg) • Ideal physical characteristics (140keV; T1/2 - 6hrs) • Readily available & cheap• Low dose to thyroid• Preferred over iodine when:
- patient on thyroid blocking agents- patient unable to take meds orally- study must be completed in < 2hrs
Dosage & Administration
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Drugs - hyperthyroidism:• Propylthiouracil • Tapazole® (methimazole)
Drugs - hypothyroidism: • LEVOTHROID®/ SYNTHROID ®
(synthetic T4)• Cytomel® (synthetic T3)
Thyroid drug abstinence before imaging
(blocks thyroperoxidase)
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123I Sodium iodide (Na123I)• Trapped & organified
• 159keV gamma emission and excellent trapping makes it ideal imaging agent with low background
• Cyclotron produced (↑ cost, problems with availability & delivery)
• 200-600 μCi capsules
• T1/2 - 13 hrs; maximum uptake at 24hrs
Radiopharmaceuticals usedDosage & Administration
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131I Sodium Iodide (Na131I)
• Same uptake principles as Na123I
• Used for imaging (50 – 200 μCi) & therapy
• β emission & T1/2 of 8 days - ↑ radiation dose to thyroid (disadvantages for imaging/ advantages for therapy followed by delayed imaging)
• 364keV gamma emission used for imaging
• High energy collimator required
• ↓ cost & readily available
medic.usm.my
Administered as solution (Tx) or capsules (Dx/Tx)
Radiopharmaceuticals usedDosage & Administration
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Tc-99m pertechnetate
I-123 sodium iodide
I-131 sodium iodide
Activity 2-10 mCi 200-600 μCi 50-200 μCi
Route IV Oral Oral
Localization Trapped but not organified
Trapped and organified
Trapped and organified
Time to Imaging 20 mins – 1 hr 4 – 24 hrs 4 – 24 hrs
Half-life 6 hrs 13 hrs 8 days
Gamma energy 140 keV 159 keV 364 keV
Radiation dose 0.13 rad/mCi 0.007 rad/μCi 1-3 rads/μCi
Thyroid Imaging Radiopharmaceuticals PropertiesRadiopharmaceuticals used
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1) ID patient; verify physician’s order; review clinical indication for thyroid imaging
2) Explain procedure to patient; obtain relevant medical history
Technique
Clinical Procedure
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Relevant Medical History
• Physical findings (neck palpation, vital signs)
• Symptoms of hyper/hypothyroidism
• Medications/dietary supplements/birth control pills
• Surgery, esp neck, upper chest
• Malignancies• Changes in ability to
swallow/voice• Previous medical
imaging• Lab values: thyroid
hormone levels• Pregnancy/lactation
– TcO4-: Stop for 12h– I-123: Stop for 3d– I-131: Stop for 3m
Technique
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Symptoms of Hyper/HypothyroidismHyperthyroidism• Nervousness• Palpitations• Diarrhea• Sweating• Increased appetite• Heat intolerance
Either• Fatigue• Dyspnea• Weight change up or
down
Hypothyroidism• Coarse hair• Puffy eyelids• Dry skin• Myxedema• Constipation• Paresthesia• Decreased appetite• Cold intolerance
Technique
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Clinical Procedure (cont’d)
3) Prepare patient- ensure that female patients are not pregnant or breast feeding (if breast feeding provide instructions)- rule out substances that may affect radioiodine uptake into thyroid gland
Technique
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Factors Influencing Thyroid Uptake of Iodine
Decreased Uptake• Iodine-rich foods (shellfish)• Radiographic contrast containing iodine• Iodine-containing medications (vitamin/mineral
supplements, cough medicines, certain skin ointments)• Non-iodine containing medications (penicillin, steroids,
antithyroid drugs)• Thyroid hormones
Technique
Steroid Tyrosine
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Factors Influencing Thyroid Uptake of Iodine
Increased Uptake• Iodine deficiency• Pregnancy• Renal failure
Technique
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Clinical Procedure (cont’d)4) Administer radiopharmaceutical
5) Image patient
Imaging time post tracer administration:Tc-99m pertechnetate: 15-30 minI-123 sodium iodide: 4-6 hrs/16-24 hrsI-131 sodium iodide: 16-24 hrs
Patient positioning: supine with neck hyperextended
Views: anterior, obliques
Pinhole collimator
Mark anatomical landmarks
Technique
Pinhole collimator
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Marking anatomical landmarks
• supra sternal notch• thyroid cartilage• chin (approx.)
Technique
One or more landmarks and palpable nodules may be marked with the help of a radioactive/ radioopaque source:
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The Normal Scan
Right Lobe Left Lobe
IsthmusNormal:
Butterfly-shaped gland with uniform, symmetrical tracer distribution
Normal radioiodine Image
Normal pertechnetate image
Chin
SSN
Anterior w/ markers
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THYROID
Non-Imaging Procedures
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Thyroid Uptake Study
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Radioiodine Uptake Study (RAIU)
• A measure of thyroid function(hyperthyroid, hypothyroid, euthyroid)
• What % of the administered radioiodine is taken up by the thyroid gland?
(Thyroid uptake can also be determined using i.V. administered 99mTcO4
- & gamma camera but is less preferred)
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Clinical Procedure1) At 4-6 hrs and/or 24 hrs following radioiodine
administration, collect counts over the following areas using an uptake probe:
patient’s neckpatient’s thighstandard (neck phantom)room background
neck phantom
Standard Capsule Method
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Thyroid Neck Phantom• designed to simulate a patient’s
neck • made of lucite• Has two part insert that allows
counting from a bottle, vial or capsule
• capsule holder enables counting capsules directly
• phantom’s cylinder and carrier have scribelines for accurate alignment
• flat surface on the cylinder allows either vertical or horizontal positioning
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Clinical Procedure (cont’d)
2) Place all counts in counts/minute (cpm). Calculate net patient and standard counts by subtracting the appropriate background counts.
CountsC1
(cpm)
CountsC2
(cpm)
Average(C1+C2)/2
(cpm)
Net(Avg – Bkg)
(cpm)
Capsule counts 90 110 100 100 – 10 =
90
Room Bkg* 9 11 10
Neck counts @
4 hrs40 50 45 45 – 25 =
20
Thigh bkg* counts @
4 hrs 30 20 25
Neck counts @
24 hrs 60 70 65 65 – 20 =
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Thigh bkg* counts @
24 hrs15 25 20
* Bkg - Background
Sample uptake work sheet
Standard Capsule Method
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Uptake calculation3) Calculate the % radioiodine uptake using the
following formula:
% uptake = neck cpm - thigh cpm x100 standard cpm - room bkg cpm
= Net neck cpm x 100 Net capsule cpm
Standard Capsule Method
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ExampleThe following data were collected 24 hrs afteradministration of two 100 μCi I-123 capsules.
Avg Counts Time (min) cpmNeck 15380 5 3076Thigh 860 5 172Standard 12600 2 6300(1 capsule)
Room bkg 350 5 70
Standard Capsule Method
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% uptake = neck cpm - thigh cpm . x 100 standard cpm - room bkg cpm
= 3076 cpm - 172 cpm . x 100 2(6300 cpm – 70 cpm)
= 23%
Standard Capsule Method
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Avg. counts CPM Net counts
Capsule(Std.1+Std.2)
12600 + 12600 25200 / 2 min
12600 12530
RoomBKG
350 / 5min 70
Neck 15380 / 5min 3076 2904Thigh 860 / 5min 172
Net Neck cpm X 100
Net Capsule cpm% Thyroid Uptake =
at 24hrs
2904 X 100
12530
= 23%
Standard Capsule Method
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Normal Ranges(may vary from dept to dept)
4 hr uptake 6 - 18%
24 hr uptake 10 - 35%
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Using same capsule for both patient and standard
• Count capsule with uptake probe before administering to patient; note counts and time
• When patient counts are collected, decay correct initial capsule counts to be used in calculation
Net neck cpm x 100 Net capsule(s) cpm x Decay Factor
Capsule Decay Method
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Sources of Error
• Patient contains residual radioactivity from previous test/therapy
• Counting geometry must remain constant for patient and standard
• Elevated room background
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Perchlorate Discharge Test
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Why is it done?
• Used to evaluate any organification defect – thyroid traps iodide but it does not combine with tyrosine
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Clinical Procedure1) Patient receives radioiodine capsule
2) Two hrs following radioiodine administration, a baseline uptake is performed. Then potassium perchlorate (KClO4) is administered orally.
3) Sixty to ninety minutes later, another thyroid uptake is performed.
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Interpretation of Results
• If an organification defect is present, the perchlorate ion (ClO4
-) displaces the iodide ion (I-) that has not been organified and the post-perchlorate uptake will be lower (10-15%) than the initial value.
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