Sarah M. A. Caney BVSc PhD DSAM(Feline) MRCVS

RCVS Specialist in Feline Medine

Hyperthyroidism is a common condition, especially of older cats. Although many cases are straightforward to diagnose and treat, this is not always the case. What follows is the second part of an article that aims to answer some of the more commonly asked questions about the diagnosis and management of feline hyperthyroidism.

How do I manage concurrent diabetes mellitus and hyperthyroidism?

In most cases where both of these diseases are present concurrently, one condition develops before the other and therefore should be stabilized first.

If a cat develops hyperthyroidism first then in most situations it will be stabilized before the cat subsequently develops diabetes mellitus. Management of the hyperthyroidism does not need to be changed and the clinician should focus on treatment of the diabetes mellitus in these cats.

Although many antithyroid medications contain sugar, the author does not consider this to be a significant contra-indication to their use in cats suffering from concurrent diabetes mellitus.

If a well-controlled diabetic cat develops hyperthyroidism then typically their diabetic control will deteriorate leading to a return of clinical signs such as polydipsia/polyuria and weight loss. A higher dose of insulin is typically needed to maintain diabetic control.

Assessment of fructosamine levels is less helpful as an indicator of diabetic control in cats with uncontrolled hyperthyroidism since accelerated protein turnover leads to a reduction in levels of fructosamine – in other words the fructosamine levels of a cat with concurrent diabetes mellitus and hyperthyroidism are often lower than expected (and may be within the reference range).

Following treatment of the hyperthyroidism, the insulin dose may need to be decreased so care should be taken to monitor patients for evidence of hypoglycaemia. Presence of diabetes does not have any impact on medications used for hyperthyroidism so these can be used at their standard doses.

How do I manage concurrent chronic kidney disease (CKD) and hyperthyroidism?

IAll treatments for hyperthyroidism have the potential to worsen kidney function. This is because the hyperthyroid condition increases renal blood flow and glomerular filtration rate (GFR). When the hyperthyroidism is treated, the increased blood flow to the kidneys decreases and GFR may fall by up to 50%.

In spite of these concerns, for most patients, optimal management of concurrent hyperthyroidism and CKD is still possible. Optimal management of hyperthyroidism is highly desirable since hyperthyroidism causes renal damage, worsening the CKD.

In a cat known to have CKD, treatment for hyperthyroidism may worsen renal function although typically, it is only cats with very serious CKD (e.g. IRIS Stage 4, creatinine > 440 μmol/l) where optimal management of hyperthyroidism proves difficult/impossible without inducing a clinical and laboratory deterioration in renal function. For this reason, medical treatment of hyperthyroidism is often recommended initially since this is a reversible treatment which can be reduced or stopped if problems are seen.

An iodine-restricted food is not recommended as the management option for cats with significant renal disease (IRIS Stage 3 or 4, creatinine > 250 umol/l) since a renal diet is indicated for these patients. When using antithyroid medication, it is prudent to start at a low dose – for example 1.25 – 2.5 mg methimazole/thiamazole per day. If tolerated but insufficient to control the hyperthyroidism, the dose can be increased as needed.

Is there any way I can predict which patients will develop renal complications?

Unfortunately there is no proven way of predicting which cats will suffer a clinically significant deterioration in renal function following treatment of their thyroid disease. The deterioration seen may worsen pre-existing renal disease or reveal (‘unmask’) renal disease that was not previously known about. Published studies have shown that assessment of pre-treatment creatinine, urine specific gravity, proteinuria and other laboratory parameters is not reliable in predicting which patients will develop renal complications. GFR assessment may offer some advantages but this is an expensive test to perform.

The fall in GFR following achievement of euthyroidism is typically stable within 4 weeks.

If renal complications develop following antithyroid medication, what should I do?

If possible the hyperthyroidism should be treated optimally (i.e. reducing total T4 to the lower half of the reference range if using antithyroid medication). However, if a clinical and laboratory deterioration in renal function is experienced then the dose of antithyroid medication should be reduced, for example reducing the dose of methimazole/thiamazole by 2.5 mg per day initially.

Where renal disease is diagnosed, other treatments for this such as feeding a specially formulated renal diet are also important to ensure the best long-term outcome for the patient.

What is iatrogenic hypothyroidism and do I need to worry about it?

Yes, new data suggests that iatrogenic hypothyroidism (IH) is more common than previously thought and that, when present, it is associated with a worse prognosis due to increased likelihood of renal complications. Clinical signs of IH are not always obvious but include lethargy, weight gain, seborrhoea sicca and alopecia. Routine lab profiles may reveal hypercholesterolaemia, mild non-regenerative anaemia and azotaemia.

How is iatrogenic hypothyroidism diagnosed and managed?

Cats receiving anti-thyroid medication/ iodine restricted food:

In cats receiving reversible treatment for their hyperthyroidism (e.g. antithyroid medication, iodine-restricted food), the aim should be for total T4 levels to be in the lower half of the reference range. The dose of antithyroid medication should be adjusted to achieve this aim. If T4 results are below this level, clinicians should consider reducing the dose of antithyroid medication or withdrawing the iodine-restricted food. It is important to remember that concurrent disease can suppress T4 levels so a low total T4 is not diagnostic for IH. If in doubt, IH can be confirmed either by measuring endogenous TSH levels (elevated in a cat with IH) or performing a TSH or TRH stimulation test (cats with IH fail to respond adequately to TSH or TRH).

Cats that have had radioiodine or surgical thyroidectomy:

In cats that have received curative treatments for their hyperthyroidism, IH may develop as a transient or permanent complication. The total T4 should be checked at 1, 3 and 6 months following treatment. If T4 levels are low and especially if azotaemia is present, then confirmation of IH is recommended by measuring endogenous TSH levels (elevated in a cat with IH) or performing a TSH or TRH stimulation test (cats with IH fail to respond adequately to TSH or TRH). If IH is confirmed and especially if azotaemia is present, then supplementation with L-thyroxine is recommended, as discussed below.

If T4 levels are low but no azotaemia is present then the patient can be monitored initially to see if the IH is transient.

Thyroid hormone supplementation (L-thyroxine at an initial dose of 0.1 mg orally once or twice daily) is recommended in confirmed cases of IH, especially in those cats that are azotaemic or showing clinical signs of hypothyroidism. The dose is adjusted according to clinical response, total T4 (4 hours post pill) and endogenous TSH levels.

Make sure to read part one of this article which was published in June. And don’t lose sleep over those complex cat cases when our feline specialist is only a call away!