Radioactive iodine treatment information for hyperthyroidism: For referring veterinarians

By Dr Joanna White

Confirming the diagnosis of hyperthyroidism

Cats referred for radioactive iodine treatment should have a total T4 measured at an external laboratory. Free T4 measurements can be high in normal cats and in-house tests for T4 can also be less reliable. So while these tests can help in individual cats, the definitive diagnosis for radioactive iodine treatment requires an external TT4. Commercial canine TSH tests are ineffective for diagnosing feline hyperthyroidism due to poor sensitivity at lower value ranges in normal cats. This may change in the future with the development of a commercial feline TSH.

Kidney function

There remains a common misconception that persistent hyperthyroidism “protects” kidney function. A more accurate representation is that hyperthyroidism masks the true extent of kidney dysfunction and probably causes progressive renal injury as well as the expected complications from untreated hyperthyroidism.

Azotaemia post radioactive iodine treatment can be due to iatrogenic hypothyroidism, and we do our best to avoid this complication. However, some euthyroid cats will develop azotaemia post treatment. This occurs because the GFR is increased in the hyperthyroid state and the cat’s true renal function can only be evaluated once GFR normalises and cats are euthyroid. There can be some increase with increased muscle mass once the hyperthyroidism has resolved.

All cats referred for radioactive iodine should have kidney function assessed (serum creatinine at a minimum).  The ideal time to assess kidney function before radioactive iodine is when cats are euthyroid (lower 50% of the reference range TT4). However, for cats that are non azotaemic (serum creatinine < 140 umol/l) when hyperthyroid, trial treatment with medical management is not essential before referral for radioactive iodine. Cats that are not azotemic when hyperthyroid but develop azotaemia after treatment typically have a good prognosis, provided they do not become hypothyroid simultaneously.

While a normal creatinine concentration in a hyperthyroid cats cannot confirm normal kidney function, cats that are azotemic when hyperthyroid are likely to have significant kidney disease, the true extent of which will be unmasked once they are euthyroid. These cats can be treated with radioactive iodine only if their owners are aware of the presence CKD and its associated prognosis.

SDMA does not consistently correlate with creatinine or GFR in cats with hyperthyroidism and in some cats, SDMA will increase post hyperthyroid treatment.  Factors other than GFR may affect serum SDMA in hyperthyroid cats.

Discontinuing anti thyroid medications

For cats with mild to moderate hyperthyroidism, discontinue anti-thyroid medications and Y/D diets 5 days before the appointment.

For patients with severe hyperthyroidism causing cardio-respiratory complications or weakness, discontinuing anti-thyroid medications for two weeks can be unsafe. For cats with very severe clinical signs, medication can be continued until the morning of their SASH appointment. Please contact us for advice if you are concerned this applies to your patient.

Cats that are not suitable for radioactive iodine treatment

Most cats are suitable candidates for radioactive iodine treatment

The few exceptions would include:

1. Cats that cannot be handled.  Many older cats and cats with hyperthyroidism can be a little grumpy, resent venipuncture and are too smart to be easily fooled. We are experienced handling these cats. The only cats we cannot treat are the small number that will not allow their cages to be cleaned or food to be replenished without attacking the SASH team.  Pre-treating cats with gabapentin on the day of their appointment with us can be helpful.
2. Cats with significant concurrent medical problems. If cats become ill after they are injected, we cannot attend to them without being exposed to high levels of radioactivity. Therefore, we cannot take cats that are known to have other serious concurrent problems. Medications can be put in the cat’s food, but the cats cannot be given tablets directly.

Post radioactive iodine follow up

Cats should be reassessed with complete physical examination including body weight, blood pressure, external TT4 and TSH and renal function tests (minimum creatinine concentration) at 6 weeks and 6 months after treatment then yearly as older cats. For cats with complicated medical histories or concern regarding their kidney function, we will recommend more frequent rechecks.

Although we can see the cats at the SASH for these appointments, most owners prefer to return to the referring veterinarian for this follow up. We appreciate any updates that you can provide. This follow-up is useful for us to review and assess our service. If the cat develops hypothyroidism, information regarding treatment is below. However, if this is not clear, or you would like to consult with us about the case, then please contact the client services team for advice from the internal medicine team. We will not consult directly with owners regarding these results as they are best interpreted in line with your physical examination and assessment of patient history.

Most cats have normal or subnormal total T4 measurements 6 weeks after radioactive iodine treatment. If the T4 is low at this time we would not recommend taking any action unless the patient is azotaemic because T4 will often return to normal over the next few months.

For cats that still have increased T4 at 6 weeks we would recommend re-measuring total T4 in one month as T4 can normalise in most cats over time unless it is very high (T4 > 150 nmol/L). If the TT4 is > 150 nmol/L at the 6 week mark, please contact SASH as we are likely to advice a repeat treatment.

Cats that are azotaemic following treatment should have a TSH is measured to evaluate for iatrogenic hypothyroidism. Iatrogenic hypothyroidism is a relatively common complication of treatment with radioactive iodine.

There are different cut offs for the diagnosis of hypothyroidism. Hypothyroidism is diagnosed when total T4 is low and TSH is high, but cut offs differ in the literature. In practice, most cats with iatrogenic hypothyroidism have a convincingly low T4 and high TSH.

A.

B.

T4 < 20 nmol/L and TSH (canine) > 0.9 ng/mL consistent with iatrogenic hypothyroidism.

Iatrogenic hypothyroidism

In cats that are not azotaemic, we do not recommend starting levothyroxine replacement based on a single high TSH or low T4 because pituitary-thyroid function will normalise in about a third of cats. However, these cats need close monitoring of their TSH, T4 (external) and creatinine concentrations every 1-3 months and may require thyroid supplementation.

If a low serum T4 and high TSH concentrations persist, or the cat is newly azotaemic, levothyroxine is recommended. Supplemental levothyroxine replacement for non-azotaemic cats should also be considered if serum TSH concentrations remain very high (>0·9 ng/mL) and if other clinical signs (e.g. lethargy, hair loss) develop.

Most cats require a total dose of 50 to 100 µg BID of levothyroxine. Once normal T4 is established, renal function can be accurately assessed. For some cats, the thyroxine can be slowly weaned off over the subsequent few months. If cats are not treated with thyroxine, their recheck T4, creatinine, and TSH should be rechecked monthly to confirm they become euthyroid.

Sub-clinical hypothyroidism

Subclinical hypothyroidism is diagnosed in cats with low to normal total T4 with high TSH concentration

If the cat is non-azotaemic, we recommend monitoring them every 3 months. If the cat is azotaemic then supplementation with levothyroxine as described above would be recommended.