1. Radioiodine Treatment for Benign Thyroid Diseases
Teofilo O.L. San Luis, Jr., MD, MPA
Professor, Faculty of Medicine & Surgery, University of Santo Tomas
Chief, Section of Nuclear Medicine, UST Hospital, Manila, Philippines
Dean, Asian School of Nuclear Medicine

2. I131 Treatment
Indicated for Hyperthyroidism (Graves’ Disease, Toxic Nodular Goiter), recurrent disease, those with contraindications to anti-thyroid drugs & surgery
Not indicated for:
“Destruction-induced thyrotoxicosis”
Thyroiditis (acute, subacute, postpartum)
Thyrotoxicosis factitia
Hyperthyroxinemia
Used in shrinking euthyroid goiters or nodules

3. What happens inside the Thyroid Follicular Cell?
Steps in Thyroid Hormone Biosynthesis
Iodide trapping by the thyroid follicular cells
Translocation of iodide to the apex of the cells
Transport of iodide into the colloid
Oxidation of inorganic iodide to an inorganic form (I+) and incorporation of iodine into tyrosine residues within thyroglobulin molecules in the colloid
Combination of two diiodotyrosine (DIT) molecules to form tetraiodothyronine (thyroxine,T4) or of monoiodotyrosine (MIT) with DIT to form triiodothyronine (T3)
Uptake of thyroglobulin from the colloid into the follicular cell by endocytosis, fusion of the thyroglobulin with a lysosome, and proteolysis and release of T4, T3, DIT and MIT
Release of T4 and T3 in the circulation
Deiodination of DIT and MIT to yield tyrosine
***T3 is also formed from monodeiodination of T4 in the thyroid and in the peripheral tissues
REFERENCE
Modified from Scientific American Medicine, Scientific American, New york, 1995

4. What happens inside the Thyroid Follicular Cell with I131?
Steps in Thyroid Hormone Biosynthesis
Iodide trapping by the thyroid follicular cells
Translocation of iodide to the apex of the cells
Transport of iodide into the colloid
Oxidation of inorganic iodide to an inorganic form (I+) and incorporation of iodine into tyrosine residues within thyroglobulin molecules in the colloid
Combination of two diiodotyrosine (DIT) molecules to form tetraiodothyronine (thyroxine,T4) or of monoiodotyrosine (MIT) with DIT to form triiodothyronine (T3)
Uptake of thyroglobulin from the colloid into the follicular cell by endocytosis, fusion of the thyroglobulin with a lysosome, and proteolysis and release of T4, T3, DIT and MIT
Release of T4 and T3 in the circulation
Deiodination of DIT and MIT to yield tyrosine
***T3 is also formed from monodeiodination of T4 in the thyroid and in the peripheral tissues
REFERENCE
Modified from Scientific American Medicine, Scientific American, New york, 1995

5. I131 Treatment
I131 emits beta rays (0.61 MeV); max range in tissue = 2.4mm
Beta rays  destruction of thyroid follicular cells  decreased thyroid cell mass  reduced thyroid hormones over a period of time
Variability in
how long a time is the effect of I131?
how much cell mass is destroyed?
how much thyroid hormone can still be produced?

6. Factors affecting I131 effectiveness:
Dose itself
Over-all gland size (or weight)
Uptake into the thyroid
Transit through the thyroid
Status of iodine sufficiency (or deficiency)
“Radiation sensitivity”

7. FACTOR # 1: DOSE ITSELF Dose Calculation
Dose = Gland Weight x uCi/gm x 100
% Uptake (24 hrs)
Considerable variation in any of the entries
Fixed Dose vs. Calculated Dose vs. Empirical Dose 5 mCi; 10 mCi; 15 mCi
100 – 160 uCi / gm
5,000 – 15,000 rads or 50 – 150 Gy
Physician subjective factors: clinical contingencies, urgency for treatment, severity of disease, etc.)
Regulatory requirements: out-patient vs. in-patient

8. FACTOR # 2: GLAND SIZE (WEIGHT)
Variability in gland size (weight estimation)
Clinical palpation estimate (based on “experience”)
Thyroid scan image estimate (“eye-balling”)
Thyroid scan planimetry
RL = Length x Width, e.g. 6 cm x 2.5 cm = 15.0
LL = Length x Width, e.g cm x 3 cm = 16.5
31.5
31.5 cm2 x “constant” (thickness) “1.44” = cm3 or 45 gms
Ultrasound volume (machine-derived)
RL = Length x Width x Depth x “correction factor”
LL = Length x Width x Depth x “correction factor”
6 x 2.5 x 2.5 x “0.52” = 19.5
5.5 x 3 x 2.5 x “0.52” = 21.5
41 gms

9. FACTOR # 3: UPTAKE INTO THE ORGAN
Hyperthyroid uptake ranges from 30 – 90%
Distribution: uniform vs. non-uniform
24 hr RAIU & scan as appropriate indicators
Therapy uptake decreased by iodine contaminants (food, drugs)
FACTOR # 4: TRANSIT THROUGH THYROID (Biologic half-life)
Hyperthyroid patients: very rapid thyroidal turnover of iodine = short biologic half-life
Depleted thyroidal iodine pool (in 15% of pts)
Rapid transit  decreased thyroid radiation dose (but increased blood & marrow radioactivity)  effectiveness?

10. FACTOR # 5: STATUS OF IODINE SUFFICIENCY (or DEFICIENCY)
Intra-thyroidal iodine stores
In iodine sufficient areas: – 20 mg (mean: 15 mg)
In iodine deficient areas: – 5 mg (mean 3.5 mg)
Competition of stable iodine vs. radioiodine
“Iodine-poor” diet list, etc
FACTOR # 6: RADIATION SENSITIVITY Difficult to measure
Prior use of anti-thyroid drugs (PTU but not Methimazole) confers “radio-resistance”  treatment failure vs use higher I131 dose

11. I131 Treatment Outcomes Effects on Thyroidal Physical Configuration
Decrease in global size (for diffuse enlargement)
Decrease in nodule number & dimensions (for nodular enlargement)
Effects on Thyroidal Function
Desired: Euthyroidism
Inevitable: Hypothyroidism
Unintended: Persistent hyperthyroidism
Unfortunate / Inadvertent: Thyroid storm

12. I131 Treatment Outcomes
Clinical Thyroidal Outcomes Hypothyroidism: early (within 1st year): 20%; 2 – 3% / annum; 50% within decade; % every 2 decades
Clinical Non-Thyroidal Outcomes
Risk for carcinogenesis: very little
Other effects: hardly, if at all
Non-Clinical Outcomes
Safety & Efficiency
Economic (Cost-Benefit)

13. I131 Treatment: Summary
I131 has definitive role in the management of hyperthyroidism and other benign thyroid conditions, as appropriate
Radiation effects influenced by several factors
Effects on thyroidal physical configuration & function with projected variable outcomes
I131 therapy is safe, efficient, cost-beneficial

14. Thank you!
Salamat Po!
Muchas gracias!
Merci beaucoup!
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Terimah kasih!
Shoukran!
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Mamnun!
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Arigato gozaimas!
Xie xie!
Mesi!
Kab Khun!
Dank!