Radioactive Iodine Treatment for 
Hyperthyroidism in a Domestic 
Shorthair Cat
Alexis Deyo
4/17/2019
Clinical Advisor: Dr. Adam Miller
Basic Sciences Advisor: Dr. Mark Roberson
Introduction
 13 year old, male neutered, domestic shorthair
● Indoor/outdoor
● Diet: Friskies canned cat food 1.5 cans daily
● Historically healthy with no chronic illness
https://www.catster.com/cats-101/tabby-cat
rDVM Visit #1: November 2018
2 month history of polyuria, polydipsia, increased activity level,  
polyphagia & weight loss 
Physical exam: Thin body condition (BCS 3/9)  and muscle wasting
Diagnostics:  Total T4 (thyroxine), Urine culture, Blood chemistry, SDMA
Results:  
⇧ (T4)= 9.1   (reference range: 2-5 ug/dL)
SDMA, BUN, Phosphorus→ High end of normal*
*hyperthyroidism → ⇧ Glomerular filtration Rate (GFR)→ masks underlying Chronic Kidney 
Disease (CKD)
DDX: Polyuria and Polydipsia with Weight Loss
❖ Kidney Pathology ❖ Diabetes Mellitus
Examples:
❖ Hyperadrenocorticism
➢ Chronic Kidney Disease
❖ Hypoadrenocorticism
➢ Chronic Pyelonephritis
❖ Hypercalcemia ❖ Hyperthyroidism
❖ Neoplasia 
❖ Exocrine Pancreatic 
Insufficiency
DDX: Polyuria and Polydipsia with Weight Loss
❖ Kidney Pathology ❖ Diabetes Mellitus
Examples:
❖ Hyperadrenocorticism
➢ Chronic Kidney Disease
❖ Hypoadrenocorticism
➢ Chronic Pyelonephritis
❖ Hypercalcemia ❖ Hyperthyroidism
❖ Neoplasia 
❖ Exocrine Pancreatic 
Insufficiency
DDX: Polyuria and Polydipsia with Weight Loss
❖ Kidney Pathology ❖ Diabetes Mellitus
Examples:
❖ Hyperadrenocorticism
➢ Chronic Kidney Disease
❖ Hypoadrenocorticism
➢ Chronic Pyelonephritis
❖ Hypercalcemia ❖ Hyperthyroidism
❖ Neoplasia 
Diagnosis: 
❖ Exocrine Pancreatic CKD (⇧ BUN, Phosphorus, SDMA)
Insufficiency Hyperthyroidism (⇧ T4)
⇨Started on oral Methimazole
Methimazole: Mechanism of Action
● Anti-thyroid drug
● Inhibits formation of thyroid hormones
Inhibits thyroid peroxidase activity
 Iodine into tyrosyl residues of thyroglobulin
Methimazole 
Advantages Adverse Effects 
● Low cost     ~25% of cats
● >95% response rate
● Facial Pruritus
Disadvantages ● Hepatotoxicity
● Daily medication ● Vomiting/ Inappetence
● Frequent lab tests ● Neutropenia/ 
● Non-curative; thyroid tumor thrombocytopenia
continues  to grow ● Temporary hypothyroidism
rDVM Visit #2: January 2019
the patient returns to his primary care vet in January 2019
● Owner had been unable to medicate consistently due to patient 
non-compliance
● Recheck T4: 18.1 ug/dL 
○ Worsening hyperthyroidism
→ the patient was referred to CUHA for Radioactive Iodine 
therapy (RAIT)
Feline Hyperthyroidism
Pathophysiology: Multisystemic Signalment
disorder.  Clinical signs due to increased 
circulating thyroid hormones Average age: 12-13 years
Etiology No sex predilection 
Functional adenomatous hyperplasia Siamese and Himalayan cats may 
involving one or both thyroid glands have reduced risk
● Bilateral hyperplasia (70%)
● Unilateral hyperplasia (30%)
● < 5% of cats → malignant thyroid 
carcinoma
*Randolph notes
Hypothalamic-Pituitary-Thyroid Axis
HPT axis primarily controlled 
by  negative feedback 
TSH stimulates release of 
thyroid hormones
Thyroid hormones regulate 
metabolism, growth and 
cardiovascular system
Clinical Signs of Feline Hyperthyroidism
Weight Loss Tachycardia
Polyphagia Cardiac murmur / arrhythmia 
Polyuria/ Polydipsia Tachypnea or Dyspnea (especially with stress 
Hyperactivity/ Behavior changes or restraint)
Vomiting Ventroflexion of head and neck (1-3% of cats)
Diarrhea or Voluminous Stools
 Unkempt haircoat
Potential Causes of Feline Hyperthyroidism
1. Endocrine disruptors in canned food
○ bisphenol-A (plasticizer in fish-flavored, canned cat food)
○ Polybrominated diphenyl ethers (flame retardant in household fabric)
2.   Fluctuating iodine content of commercial cat food
Thyroid Gland Anatomy
Two distinct  lobes 
Located caudal to larynx, lateral 
to trachea
Thyroid Gland Structure & Function
Functional Unit of Thyroid Gland:
Thyroid Follicles: Ring of 
epithelial cells
-produce thyroid hormones
Colloid: Intrafollicular material 
containing thyroglobulin (TG) 
reservoirs
 Thyroid Hormones
★ Thyroxine (T4): inactive pro-hormone; major form 
of thyroid hormone found in blood
○ Long half-life
★ Triiodothyronine (T3): active hormone; T4 
converted to T3 within target cells
○ Short half-life
*https://www.merckvetmanual.com/endocrine-system/the-thyroid-gland/overview-of-the-thyroid-gland#
-
Diagnosing Feline Hyperthyroidism
 Persistently Elevated Thyroid Hormone level
(Total T4 or Free T4)       
＋
Clinical Signs 
 
Diagnosing Feline Hyperthyroidism:
Thyroid Hormone Levels
❖ Total T4  
➢ High specificity (no false 
positives)
➢ ⇧ total T4= Hyperthyroid!
➢ Normal total T4 ? ❖  Free T4 
➢ Not influenced by 
→ Run free T4
concurrent disease
➢ ⇧ Free T4= Hyperthyroid!
Treatment Options for Feline Hyperthyroidism 
Radioactive Iodine Kills functional Curative
thyroid cells
Oral or Transdermal Inhibits thyroid Temporary
Methimazole hormone synthesis
Dietary: Hills Y/D Iodine-restricted diet Temporary
Surgical Thyroidectomy Remove thyroid Curative
tumor
2/11/2019: 
Presentation to CUHA              
Chief Complaint: Chronic hyperthyroidism
History on Presentation:
● Increasing hyperactivity
● Worsening polyuria/ polydipsia
● Vomiting daily
● Frequent liquid diarrhea with increased urgency
● Good appetite
● Occasionally stops and coughs after running
BCS 4/9
Grade II/VI parasternal heart 
murmur
Physical 
Examination       S y m m  e t ri ca l musculature with mild, genera liz e d   m u s c le   wasting
CUHA Internal Medicine Service 
February, 2019 Asymmetrical kidneys
Thyroid Slip
Increased lung sounds bilaterally
Prioritized Problem List  2/11/19
1. Vomiting 
2. Diarrhea
3. Polyuria/ Polydipsia  
4. Coughing
5. Grade II/VI parasternal heart murmur
6. Mild muscle wasting and weight loss
7. Polyphagia
Pathophysiology of the Patient’s Clinical Signs
● ⇧ metabolic rate→ Hyperactivity, Weight loss, Polyphagia, Muscle 
wasting
● ⇧ Renal Blood Flow → Increased GFR→ Polyuria & Polydipsia
● Excess thyroid hormone acts on Chemoreceptor Trigger Zone + 
Rapid overeating→ Vomiting
● GI hypermotility + Increased intestinal osmotic content→ Diarrhea
● Respiratory muscle weakness + ⇧ CO2 production→ Tachypnea
● Increased catecholamine activity +  Direct cardiotoxicity→ 
Tachycardia→ Hypertrophic Cardiomyopathy  & Heart murmur
Initial Plan 2/11/19
❖ Blood work: CBC, Serum chemistry, Urinalysis, T4 (thyroxine) level, 
vitamin B-12/ cobalamin level
❖ Thoracic radiographs
❖ Blood pressure 
❖ EKG
❖ Nuclear scintigraphy
❖ +/-  Radioactive Iodine therapy
Diagnostic Results 2/11/19
Total T4: 30.70 (RR: 2-5 ug/dL) Urinalysis:
Protein: 30 (1+) mg/dL
Serum Chemistry: 
Serum (Cobalamin): 470 
↑ phosphate: 5.8 (RR: 2.6-5.5mg/dL) (RR: 900-3,000 pg/mL)
↑ALT: 137 (RR: 28-109 U/L)
↑Creatine Kinase 685 (RR: 74-386 U/L)
Thoracic Radiographs 2/11/19
Moderate, diffuse cardiomegaly
Thyroid Scintigraphy 2/11/19
Bilateral hyperplasia
     R > L
 2/12/19
 Administration of Radioactive Iodine Therapy 
(RAIT)
The next day, the patient received a 
5.0 mCi subcutaneous injection of  131I 
-given higher dose due 
to high Total T4 (30.7 mg/dL!)
Goal of RAIT: Euthyroidism after one treatment 
without causing hypothyroidism
How does Radioactive Iodine work?
Radioactive Iodine (I-131) injected into patient
I-131 enters thyroid cells via I-symporter
Emits  β  particles and  γ rays 
Damage to cellular DNA
Apoptosis and cell death
Why is Radioactive Iodine (131I) so Effective ?
★ I-131 can only enter cells via I-symporters 
○ I-symporters are specific to thyroid tissue→ FOCUSED Effects
★ I-131 only acts on adjacent cells within 2-3 millimeters → LOCALIZED 
Effects
★ I-131 has a short half-life→ SHORT isolation period
○ T ½ = 8 days
Radioactive Iodine 
Advantages Disadvantages
High up-front cost
Curative Hospitalized  for 7-10 days in isolation
Effective Post-treatment precautions
Low incidence of adverse effects (<5%)
Potential for Adverse Effects:
● Dry eyes or dry mouth
● Persistent Hyperthyroidism (2.5%)
● Hypothyroidism requiring 
supplementation (2.1%)
 
2/12-2/17:
Isolation at CUHA 
Following his RAIT, the patient remained in isolation for 5 days 
No complications
Discharged on 2/17/19
Discharge Instructions 
MONITORING
Signs of hypothyroidism RECHECK in 1 month:
Worsening renal disease ❖ Total T4
❖ Serum Chemistry 
❖ Urinalysis 
RADIATION SAFETY for 3 
weeks
❖ Radiation excreted in MEDICATION
saliva, urine, feces Cyanocobalamin: Give 250 mcg 
❖ Hold and store litter subcutaneously every 7 days
❖ No contact with pregnant 
woman or children
Prognosis for RAIT Hyperthyroid Cats 
● 95% of cats are euthyroid 3 months after treatment 
● Mean survival times for RAIT cats ranges from 2-4 years after 
treatment
Follow-up: 1 month Recheck at rDVM
Total T4 level: 0.8 (RR 0.8-4.7 ug/dL)
Serum Chemistry:
↑ BUN: 38 (RR: 16-37 mg/dL)
-No other abnormalities
Otherwise, doing well!
Cost
Exam…………………………………………………………………..$200
Lab work…………………………………………………………….$309
Hospitalization…………………………………………………$201 
Imaging……………………………………………………………….$353
Radioactive Iodine therapy………………………….$500
Medications………………………………………………………..$34
Total Cost of Visit……………………...………………$1597.11
 Selected References
1. Peterson, ME.  Radioiodine Treatment of Hyperthyroidism. Clinical Techniques in Small 
Animal Practice. 2006. 21(1): 34-9.  
2. Vaske, HE et al.  Diagnosis and Management of Feline Hyperthyroidism: Current 
Perspectives.  Veterinary Medicine: Research and Reports 2014. 5: 85-96.
3. Carney, HC et al.  2016 AAFP Guidelines for the Management of Feline Hyperthyroidism.  
Journal of Feline Medicine and Surgery. 2016. 18: 400-416.
4. Slater, MR et al.  Long Term health and Predictors of Survival for Hyperthyroid Cats 
Treated with Iodine 131.  Journal of Veterinary Internal Medicine.  2001. 15: 47-51.
5. Boron, WF. The Thyroid Gland. Medical Physiology, 2nd Edition. Saunders, Elsevier. 2012. 
Pp 1057-1059.
 Images
www.rvc.ac.uk/small-animal-referrals/advanced-techniques-and-specialist-procedures/radioactive-iodine-i131-therapy
https://www.merriam-webster.com/words-at-play/the-history-of-the-word-tabby
http://doctorsgates.blogspot.com/2011/01/synthesis-transport-of-thyroid-hormones.html
https://www.catster.com/cats-101/tabby-cat
Block 2 Notes, Dr. Roberson
Radioactive_cats_t_shirt-r4a490c914b1f45aaaef00e049ccc4b6f_k2ggc_307.jpg
https://imgur.com/gallery/dkqhf
https://www.osvs.net/specialty-services/radioactive-iodine-therapy.html
Thank You!
My Advisors: Dr. Miller & Dr. Roberson
Class of 2019
My family, friends,  and pets 
Questions?