Thyroid anatomy,physiology,thyroid scintigraphy principles

transcript

Thyroid Anatomy/Physiology/Thyroid scintigraphy principles

NMT631

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

Thyroid - Anatomy

A. Normal Adult Thyroid weighs 20-25g

B. Components

1) Two Lateral lobes 2) Isthmus centrally connects the

3) A Pyramidal lobe in 50% of the population

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

Thyroid Hormone Synthesis1) Absorption

2) Trapping

3) Organification

4) Coupling

5) Release

Absorption

• Typical diet contains 300 -1000 μg of Iodine/day

• Iodine ingested in food is reduced to iodide, then absorbed into the blood

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.

Trapping

• Iodide then trapped by follicular cells

• Trapped iodide ions are oxidized by peroxidase enzyme.

IP – Iodide Pump

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

Organification• Iodine is linked to tyrosine (an amino acid) on thyroglobulin molecule

• Iodine + tyrosine MIT & DIT

MIT = monoiodotyrosine DIT = diiodotyrosine

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

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

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

Thyroid hormone regulation

Thyroid Hormone Regulation

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.

THYROID IMAGING

Thyroid Imaging

Clinical Indications

Radiopharmaceutical used

Dosage & Administration

Technique

The Normal Scan

Artifacts & Pitfalls

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

1) Tc-99m pertechnetate

2) I-123 sodium iodide

3) (I-131 sodium iodide)

Radiopharmaceuticals used

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

Drugs - hyperthyroidism:• Propylthiouracil • Tapazole® (methimazole)

Drugs - hypothyroidism: • LEVOTHROID®/ SYNTHROID ®

(synthetic T4)• Cytomel® (synthetic T3)

Thyroid drug abstinence before imaging

(blocks thyroperoxidase)

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

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

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Administered as solution (Tx) or capsules (Dx/Tx)

Radiopharmaceuticals usedDosage & Administration

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

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

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

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

Symptoms of Hyper/HypothyroidismHyperthyroidism• Nervousness• Palpitations• Diarrhea• Sweating• Increased appetite• Heat intolerance

Either• Fatigue• Dyspnea• Weight change up or

Hypothyroidism• Coarse hair• Puffy eyelids• Dry skin• Myxedema• Constipation• Paresthesia• Decreased appetite• Cold intolerance

Technique

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

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

Factors Influencing Thyroid Uptake of Iodine

Increased Uptake• Iodine deficiency• Pregnancy• Renal failure

Technique

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

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:

The Normal Scan

Right Lobe Left Lobe

IsthmusNormal:

Butterfly-shaped gland with uniform, symmetrical tracer distribution

Normal radioiodine Image

Normal pertechnetate image

Anterior w/ markers

THYROID

Non-Imaging Procedures

Thyroid Uptake Study

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)

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

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

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

CountsC2

Average(C1+C2)/2

Net(Avg – Bkg)

Capsule counts 90 110 100 100 – 10 =

Room Bkg* 9 11 10

Neck counts @

4 hrs40 50 45 45 – 25 =

Thigh bkg* counts @

4 hrs 30 20 25

Neck counts @

24 hrs 60 70 65 65 – 20 =

Thigh bkg* counts @

24 hrs15 25 20

* Bkg - Background

Sample uptake work sheet

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

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

% uptake = neck cpm - thigh cpm . x 100 standard cpm - room bkg cpm

= 3076 cpm - 172 cpm . x 100 2(6300 cpm – 70 cpm)

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

Normal Ranges(may vary from dept to dept)

4 hr uptake 6 - 18%

24 hr uptake 10 - 35%

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

Sources of Error

• Patient contains residual radioactivity from previous test/therapy

• Counting geometry must remain constant for patient and standard

• Elevated room background

Perchlorate Discharge Test

Why is it done?

• Used to evaluate any organification defect – thyroid traps iodide but it does not combine with tyrosine

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.

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.

Thyroid anatomy,physiology,thyroid scintigraphy principles

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