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Thyroid anatomy,physiology,thyroid scintigraphy principles
- 1. Thyroid Anatomy/Physiology/ Thyroid scintigraphyprinciples NMT631
- 2. 2 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
- 3. 3 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
- 4. 4 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
- 5. 5 Thyroid Hormone Synthesis 1)Absorption 2) Trapping 3) Organification 4) Coupling 5) Release
- 6. 6 Absorption • Typical dietcontains 300 -1000 μg of Iodine/day • Iodine ingested in food is reduced to iodide, then absorbed into the blood
- 7. 7 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.
- 8. 8 Trapping • Iodide thentrapped by follicular cells • Trapped iodide ions are oxidized by peroxidase enzyme. IP – Iodide Pump
- 9. 9 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
- 10. 10 Organification • Iodine islinked to tyrosine (an amino acid) on thyroglobulin molecule • Iodine + tyrosine MIT & DIT MIT = monoiodotyrosine DIT = diiodotyrosine
- 11. 11 Coupling • With thehelp 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
- 12. 12 Release • In responseto 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. 13 TSH stimulatesiodide trap, thyroid hormone synthesis & release
- 14. 14 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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- 17. 17 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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- 19. 19 Thyroid Imaging Clinical Indications Radiopharmaceutical used Dosage& Administration Technique The Normal Scan Artifacts & Pitfalls
- 20. 20 1) Relate structureto function 2) Evaluation of thyroid nodules 3) Location of ectopic tissue 4) Follow-up exams (post therapy, post surgery) Clinical Indications
- 21. 21 1) Tc-99m pertechnetate 2)I-123 sodium iodide 3) (I-131 sodium iodide) Radiopharmaceuticals used
- 22. 22 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
- 23. 23 Drugs - hyperthyroidism: •Propylthiouracil • Tapazole® (methimazole) Drugs - hypothyroidism: • LEVOTHROID®/ SYNTHROID ® (synthetic T4) • Cytomel® (synthetic T3) Thyroid drug abstinence before imaging (blocks thyroperoxidase)
- 24. 24 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 used Dosage & Administration
- 25. 25 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 used Dosage & Administration
- 26. 26 Tc-99m pertechnetate I-123 sodium iodide I-131 sodium iodide Activity2-10 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 Properties Radiopharmaceuticals used
- 27. 27 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
- 28. 28 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
- 29. 29 Symptoms of Hyper/Hypothyroidism Hyperthyroidism •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
- 30. 30 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
- 31. 31 Factors Influencing Thyroid Uptakeof 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
- 32. 32 Factors Influencing Thyroid Uptakeof Iodine Increased Uptake • Iodine deficiency • Pregnancy • Renal failure Technique
- 33. 33 Clinical Procedure (cont’d) 4)Administer radiopharmaceutical 5) Image patient Imaging time post tracer administration: Tc-99m pertechnetate: 15-30 min I-123 sodium iodide: 4-6 hrs/16-24 hrs I-131 sodium iodide: 16-24 hrs Patient positioning: supine with neck hyperextended Views: anterior, obliques Pinhole collimator Mark anatomical landmarks Technique Pinhole collimator
- 34. 34 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:
- 35. 35 The Normal Scan RightLobe Left Lobe Isthmus Normal: Butterfly-shaped gland with uniform, symmetrical tracer distribution Normal radioiodine Image Normal pertechnetate image Chin SSN Anterior w/ markers
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- 38. 38 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)
- 39. 39 Clinical Procedure 1) At4-6 hrs and/or 24 hrs following radioiodine administration, collect counts over the following areas using an uptake probe: patient’s neck patient’s thigh standard (neck phantom) room background neck phantom Standard Capsule Method
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- 41. 41 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
- 42. 42 Clinical Procedure (cont’d) 2)Place all counts in counts/minute (cpm). Calculate net patient and standard counts by subtracting the appropriate background counts. Counts C1 (cpm) Counts C2 (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 hrs 40 50 45 45 – 25 = 20 Thigh bkg* counts @ 4 hrs 30 20 25 Neck counts @ 24 hrs 60 70 65 65 – 20 = 45 Thigh bkg* counts @ 24 hrs 15 25 20 * Bkg - Background Sample uptake work sheet Standard Capsule Method
- 43. 43 Uptake calculation 3) Calculatethe % 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
- 44. 44 Example The following datawere collected 24 hrs after administration of two 100 μCi I-123 capsules. Avg Counts Time (min) cpm Neck 15380 5 3076 Thigh 860 5 172 Standard 12600 2 6300 (1 capsule) Room bkg 350 5 70 Standard Capsule Method
- 45. 45 % 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
- 46. 46 Avg. counts CPMNet counts Capsule (Std.1+Std.2) 12600 + 12600 25200 / 2 min 12600 12530 Room BKG 350 / 5min 70 Neck 15380 / 5min 3076 2904 Thigh 860 / 5min 172 Net Neck cpm X 100 Net Capsule cpm % Thyroid Uptake = at 24hrs 2904 X 100 12530 = 23% Standard Capsule Method
- 47. 47 Normal Ranges (may varyfrom dept to dept) 4 hr uptake 6 - 18% 24 hr uptake 10 - 35%
- 48. 48 Using same capsulefor 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
- 49. 49 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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- 51. 51 Why is itdone? • Used to evaluate any organification defect – thyroid traps iodide but it does not combine with tyrosine
- 52. 52 Clinical Procedure 1) Patientreceives 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.
- 53. 53 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.