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

Abstract

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;