Hopkinton Animal Hospital
For Referring Veterinarians
To provide the best medical service possible to pets with feline hyperthyroidism and their owners, we require a preliminary work-up on each patient prior to treatment. A clear picture of the overall health of each cat is critical in determining if it is an appropriate candidate for the treatment.
I-131 works best in the stable hyperthyroid cat without other significant diseases (renal, gastrointestinal, cardiovascular, other endocrinopathies). If your patient has a significant other disease, please contact us to discuss whether I-131 is appropriate for your patient.
Pre-Therapy Workup
You are responsible for performing pre- and post-treatment testing. We are happy to provide consulting with you prior to treatment.
Please provide the following:
Complete blood count, biochemical profile including BUN, creatinine, ALT, SAP, Na, K and urinalysis (within 60 days of admission)
T4 (or fT4) within 30 days of admission
Lateral and DV (or VD) thoracic radiographs
If the patient is currently on methimazole — and has been for more than 90 days — we require a T4 taken 7 days after cessation of this medication. If you have questions, we can suggest a schedule for stopping, being retested, and arriving for therapy.
Please supply all previous T4 values, histories of biopsies, cancer, and any current medical concerns
For peace of mind of our staff, and in accordance with New Hampshire State law, cats need to be current on their Rabies vaccination.
This information can be submitted to us in any way you prefer:
Completing an online form for referring veterinarians, linked here.
Emailing the information to us at [email protected]
Faxing the information to us at 603-696-1941
Medical Information
Patients MUST be off methimazole for at least seven days prior to admission for I-131 therapy. Pets can continue Hills y/d diet if they are currently on it. Other common medications which may interfere with therapy include antihistamines, some worming medications, sulfa antibiotics, and some vitamin preparations. Possible but very rare complications of radioiodine therapy include:
Sore throat, dysphagia. This is usually transient, but a permanent voice change is possible.
Patients may become hypothyroid. Rare cases may need thyroid supplementation. Most cases resolve within 4 months post-treatment.
Worsening of azotemia. Correction of the hyperthyroid sate (by I-131, methimazole, or surgery) reduces the hypermetabolic state which decreases the glomerular filtration rate (GFR). When the GFR returns toward normal, borderline azotemic patients can worsen. It has been estimated that in the pre-treatment azotemic patients, less than 5% have a clinically significant worsening of azotemia.
There is currently no definitive test to determine whether this will happen to a particular azotemic patient. Some clinicians have recommended that pre-treatment azotemic patients be treated for 7 days with 2.5 mg methimazole PO BID and have their BUN and creatinine rechecked in 7-10 days to see if the azotemia has worsened.
If the azotemia has not worsened, then the patient should not exhibit worsening azotemia after the I-131 therapy.
If the azotemia does worsen, please contact us.
Some cats do not respond to an I-131 treatment. Most of these patients can be successfully treated with a second injection.
Post-Treatment Value Interpretation
Four to six weeks after the I-131 treatment, please obtain a new thyroid level.
If you diagnosed hyperthyroidism based on a T4 value, repeat a T4.
If you diagnosed hyperthyroidism based on a fT4 value, repeat a fT4.
Obtain a BUN and creatinine level. If you have the ability, a SDMA would be helpful but not necessary.
Interpreting Post-131 Treatment Values
| T4 or fT4 (your lab may have different normal values) | Azotemia | Interpretation |
| T4 between 1.0 and 4.0 Ft4 0.7-2.6 ng/dL (9.0-33.5 pmol/L) | None present | Pet is back to normal. Monitor T4 every 6 months. |
| T4 less than 1.0 (or fT4 < 0.7 ng/dl) but patient is clinically normal | None present | Monitor for clinical signs of hypothyroidism (significant weight gain, seborrhea, excessive listlessness.) |
| T4 less than 1.0 (or fT4 < 0.7 ng/dl) but pet is clinically showing signs of hypothyroidism | None Present | Start on levothyroxine 0.1 mg PO q24hr for 3 to 4 months and recheck. |
| T4 between 1.0 and 4.0 Ft4 0.7-2.6 ng/dL (9.0-33.5 pmol/L) | Present | Treat for azotemia. Normal CRF protocols are appropriate. Monitor T4 every 6 months. |
| T4 less than 1.0 (or fT4 < 0.7 ng/dl) but clinically normal | Present | Treat for azotemia. Normal CRF protocols are appropriate. Monitor for clinical signs of hypothyroidism (significant weight gain, seborrhea, excessive listlessness.) |
| T4 less than 1.0 (or fT4 < 0.7 ng/dl) but clinically showing signs of hypothyroidism | Present | Treat for azotemia. Normal CRF protocols are appropriate. Start on levothyroxine 0.1 mg PO q24hr for 3 to 4 months and recheck. |
For additional information, please click here to view a Powerpoint presentation.
Scheduling Your Patient
You or your client may schedule your patient’s treatment by calling us at 603-487-6447 and asking for our I-131 Coordinator.
