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ABSTRACT OF THE WEEK

The Veterinary Clinics of North America. Small animal practice
Volume 50 | Issue 5 (September 2020)

Hyperthyroidism in Cats: Considering the Impact of Treatment Modality on Quality of Life for Cats and Their Owners.

Vet Clin North Am Small Anim Pract. September 2020;50(5):1065-1084.
Mark E Peterson1
1 Animal Endocrine Clinic, New York, NY, USA. Electronic address: drpeterson@animalendocrine.com.
Copyright © 2020 Elsevier Inc. All rights reserved.

Abstract

In cats, hyperthyroidism can be treated in 4 ways: medical management with methimazole or carbimazole, nutritional management (low-iodine diet), surgical thyroidectomy, and radioactive iodine (131I). Each form of treatment has advantages and disadvantages that should be considered when formulating a treatment plan for the individual hyperthyroid cat. Medical and nutritional managements are considered "reversible" or palliative treatments, whereas surgical thyroidectomy and 131I are "permanent" or curative treatments. The author discusses how each treatment modality could be the optimal choice for a specific cat-owner combination and reviews the advantages and disadvantages of each treatment option.

Companion Notes

Overview on the treatment of hyperthyroidism in the cat

- considering the impact of treatment modality on cat’s and owner’s quality of life (QoL)

   

Summary on the disease and its treatment

- disorder involves a transition from normal thyroid tissue to hyperplasia to adenoma

(rarely, carcinoma)

- at diagnosis almost all cats will have nodular thyroid adenomas

- not hyperplasia

- disorder is progressive and underlying thyroid tumors will continue to grow

- there are 4 treatment modalities and each with effect QOL

- medical management with methimazole or carbimazole

- nutritional management (low-iodine diet)

- surgical thyroidectomy

- radioactive iodine (131I)

   

Introduction on hyperthyroidism in the cat

- common endocrine disorder affecting about 10% of senior to geriatric cats

- rule out concurrent diseases (> 95% of affected cats are > 10 years of age)

- 20-35% of cats seen before 131I therapy having at least 1 comorbidity

- cardiac, renal, and GI disease most commonly

- small or irregularly shaped kidneys should raise suspicion of "masked" CKD

- discuss any suspected or confirmed coexisting disease with owners

- so they don’t assume therapy for hyperthyroidism will resolve all signs

- antithyroid drugs inhibit the production of thyroid hormones

- these don’t stop the progression of the disease

- thus the need for monitoring and periodic increases in dosage

   

Thyrotoxic heart disease

- this becomes more common with increased severity of hyperthyroidism

- in 1 study investigating concurrent cardiac disease

- found in 37% of cats with mild hyperthyroidism

- found in 71% of cats with severe hyperthyroidism

- overt heart failure is rare

- almost always seen in cats with severe, long-standing hyperthyroidism

- many of which can no longer be controlled with antithyroid drugs

   

Treatment choices

- younger cats without concurrent disease potentially may live a long time

- definitive treatment possibly indicated (surgical thyroidectomy or 131I)

- treatment for geriatric cats with clinically significant concurrent disease

(such as CKD or neoplasia)

- antithyroid drugs or a low-iodine diet perhaps more fitting

- most hyperthyroid cats fall somewhere in between the 2 extremes above

   

Chronic kidney disease (CKD), like hyperthyroidism, is common in older cats

- both occur frequently in the same cat

- hyperthyroidism increases the glomerular filtration rate (GFR)

- increased GFR can "mask" underlying renal insufficiency

- by lowering BUN and creatinine despite mild to moderate kidney disease

- successful therapy of hyperthyroidism brings GFR to low-normal or subnormal levels

(levels expected in cats with moderate renal dysfunction)

- this results in 1 of the following:

- worsened serum kidney function tests

- apparent development of renal disease

(masked but actually already present)

- detecting masked CKD

- serum symmetric dimethylarginine (SDMA) can be helpful as adjunctive test

- suspect masked CKD if the following is present

- serum urea, creatinine, or SDMA levels are at upper end of reference

(or borderline high)

 or

- urine specific gravity is dilute (<1.035, but certainly <1.020)

- since predicting masked CKD in hyperthyroid cats is difficult

- "methimazole trial" commonly recommended

- if hyperthyroid cats do not develop worsening azotemia after T4 normalizes

(on antithyroid medication)

- concurrent CKD can be excluded

- more definitive treatment can be selected

- ≥ 3-6 months of euthyroidism needed to accurately assess renal function

- typical trial of 1-2 months does not guarantee renal function

(renal function that will deteriorate after definitive therapy)

- ~ 25% of hyperthyroid cats can be expected to develop some degree of azotemia

 (serum creatinine >2.0 mg/dL; >175 µmol/L)

- within 3-6 months of successful treatment

- azotemia is only mild to moderate in most cats

- tends to be stable for prolong periods without significantly affecting survival

(as long as euthyroidism is maintained)

- cats becoming hypothyroid after treatment have a higher prevalence of azotemia

- in 1 study on survival of hypothyroid, azotemic cats

- survival time was ½ that of hypothyroid, nonazotemic cats

- maintain total T4 within middle half of the reference interval

- to avoid overt or subclinical hypothyroidism

(low to low-normal T4 with high serum TSH)

   

Gastrointestinal disease

- in 1 study of untreated hyperthyroid cats

- most commonly identified concurrent diseases

- chronic enteropathy

- alimentary lymphoma

- vomiting reported in just under ½ of cats with hyperthyroidism

- GI signs may be due to hyperthyroidism or concurrent GI disease

- vomiting, if present, should resolve once euthyroidism is achieved

- persistent vomit after effective treatment should raise suspicion for GI disease

- among hyperthyroid cats presenting with moderate to severe diarrhea

(as a primary clinical sign)

- most have concurrent GI disease

- short-term antithyroid trial can help determine what diarrhea is due to

   

Client communication

- in 1 survey of owners

- almost 20% would have liked more details on treatment options by their vet

- over 30% would have liked more information on long-term management

   

Treatment

- nutritional therapy may difficult for owners with multiple cats

- medicating is difficult for some owners

- surgery and 131I are "definitive" treatments (abnormal thyroid tissue addressed)

- treatment is permanent

- most younger and healthy cats best treated with definitive treatments

- definitive treatments have a high initial cost

- medical and nutritional therapies can also have a high cost

- due to many months to years of ongoing monitoring

- antithyroid drugs and low-iodine diets keep circulating T4 within reference

- these treatment choices are "palliative"

- hyperthyroidism is progressive

- risk of severe signs increases with duration

- can be used to prepare cat for definitive treatment

- antithyroid drugs, in 1 report, offered as initial treatment to 92% of owners

- in 1 survey of owners, 30% were only offered an oral antithyroid

- long-term medical therapy is best reserved for the following cats

- cats of advanced age

- not expected to live long enough for a large tumor size increase

- or for it to undergo malignant transformation

- cats with moderate to severe concurrent disease

(disease expected to shorten lifespan)

- treatment of choice when owners refuse definitive treatment

- advantages include the following:

- low cost and anesthesia is not necessary

- long term control achieved in ~ 75% of cats

- disadvantages include the following:

- medical therapy is not curative

- depends on owner compliance and cat compliance

- an underlying tumor will continue to grow

- dose must increase and cat may become resistant

- owners must be informed of risk to themselves

- methimazole or carbimazole tablets shouldn’t be crushed

- increases human exposure

- drugs are potentially teratogenic

- frequency of suspected non life-threatening adverse reactions

- lethargy, GI signs (anorexia, vomiting)

- 23% with oral methimazole

- 4% with transdermal methimazole

- 33% with oral carbimazole

- mild hematologic abnormalities

(leukopenia, eosinophilia, lymphocytosis)

- 16% with oral methimazole

- not reported with transdermal methimazole

- 35% with oral carbimazole

- facial/cervical self-induced excoriations (pruritus)

- 4% with oral methimazole

- 8% with transdermal methimazole

- 12% with oral carbimazole

- generalized peripheral lymphadenopathy

- few case reports with oral methimazole

- not reported with transdermal methimazole

- not reported with oral carbimazole

- frequency of suspected life-threatening adverse reactions

- hepatopathy (icterus/anorexia)

- 3% with oral methimazole

- 4% with transdermal methimazole

- bleeding diathesis

(epistaxis, oral bleeding, prolonged clotting time)

- 3% with oral methimazole

- not reported with transdermal methimazole

- severe thrombocytopenia (platelet count < 75,000/µl)

- 3% with oral methimazole

- 8% with transdermal methimazole

- agranulocytosis

(leukopenia; total granulocyte count <500/µL) and neutropenia)

- 3% with oral methimazole

- 6% with transdermal methimazole

- anemia (including aplastic anemia)

- few case reports with oral methimazole

- not reported with transdermal methimazole

- low-iodine diet has lowest rate of treatment success

(persistent elevated thyroid hormone seen in 30% of cats)

- over 33% of cats will not eat the diet

- diet must be fed exclusively; no treats etc

- macronutrient composition is suboptimal

- especially in older hypermetabolic cats with severe muscle wasting

- takes a few weeks to lower high serum T4

- best reserved for the following situations

- definitive therapy not possible or cat can’t tolerate it

- owner unable to administer, long-term medication

- surgical thyroidectomy is an extremely effective definitive treatment

- most cats will require bilateral thyroidectomy

(65% of cats have tumors of both thyroid lobes)

- staged surgery reduced risk of iatrogenic hypoparathyroidism

- 4 weeks between surgeries

- success rate for a cure of hyperthyroidism: >90%

- T4 and T3 will decrease to normal within 24-48 hours

- fastest method to control hyperthyroidism

- common complications include the following:

- iatrogenic hypoparathyroidism and hypocalcemia

- occurs if at least 1 parathyroid gland is not preserved

- hypocalcemia can cause muscle twitching, tetany and seizures

- hypothyroidism

- unilateral surgery will cause temporary hypothyroidism in most cats

- subnormal serum T4 and T3 levels for 3-6 months

- short-term thyroid hormone replacement (L-T4) recommended

- bilateral thyroidectomy is expected to cause hypothyroidism

- within 24-48 hours

- start thyroid hormone replacement day of discharge

- persistent hyperthyroidism

- unidentified ectopic thyroid tumors present in 4% of cases

- recurrence of hyperthyroidism occurs rarely

- months to years postop

- radioiodine therapy (gold-standard treatment of choice according to most)

- success rate >95%, noninvasive and has longer survival time

- strongly consider in all newly diagnosed cats expected to live > 2-3 years

 (cats ≤ 12 years and even very healthy geriatric cats ≤ 16 years)

- best treatment for cats with ectopic thyroid tumors or thyroid carcinomas

- may not be best option if owner can’t pill their cat

- iatrogenic hypothyroidism requires daily, generally life-long L-T4

- lower 131I dose helps minimize the chance of hypothyroidism

- with a slightly lowered cure rate

- length of hospitalization ranges with 131I dose and local regulations

- cat must be isolated for a few days in special radioactive facility

- cat is radioactive for 3-4 weeks

- owners must follow radiation safety guidelines

   

Which treatment is best for a given cat

- in younger cats without concurrent disease

- definitive treatment with either surgery or radioiodine recommended

- in cats in advanced age or those with concurrent diseases

(or owners refuse either surgery or 131I)

- long term medical or nutritional management might be best

- cats with more severe hyperthyroidism and larger thyroid tumors

- they’re more likely to become resistant to antithyroid drugs or dietary therapy

- definitive treatment is best done sooner rather than later

- later cat may have a poorer overall body condition or cardiac failure

- most cats present at an intermediate age of 13-16 years

- consider the following:

- cat’s age

- presence/absence of significant concurrent disease

- severity of clinical hyperthyroidism

- size of goiter (thyroid tumor)

- availability of skilled surgeon

- access to 1311 treatment facility

- owner ability to administer tablets/transdermal medication

- cat compliance for oral or transdermal medication

- willingness of cat to eat low-iodine diet

- multicat household or outdoor cat that hunts

- immediate and long-term costs of each treatment

- potential complications of treatments

   

“Discussing the intricacies of each modality with regards to reversibility/cure, initial and long-term (ongoing) costs, treatment options, risks, costs, and outcomes is essential to end up with the best QoL for both the owner and their cat.”

Keywords
Antithyroid drug; Feline; Low-iodine diet; Methimazole; Radioactive iodine; Radioiodine; Thyroid; Thyroidectomy;

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