Download Hypothyroidism - Wendy Blount, DVM

Hypothyroidism
Wendy Blount. DVM
Thyroid Terms
• thyros – shield
• cretinism – congenital lack of thyroid
hormones
• In dogs and cats there are 2 thyroid
glands
– left and right lobes
• T4 – thyroxine
• T3 – 3,5,3-triiodothryonine
• Stored as thyroglobulin in the thyroid
follicle
Thyroid Terms
• TRH – thyrotropin releasing hormone
(hypothalamus)
– Tells the pituitary to release TSH
• TSH – thyroid stimulating hormone thyrotropin (pituitary)
– Tells the thyroid to make T4 and T3
Function of Thyroid Glands
• maintain metabolic rate and tissue
repair
• Inotropic, chronotropic effects on the
heart
• Enhance catecholamine response
• Critical to fetal development
– Especially neurologic and skeletal
Hypothyroidism Classification
• Primary hypothyroidism
– Destruction of the thyroid gland
– 80% of the gland must be destroyed
• Secondary hypothyroidism
– Decreased TSH
• Tertiary hypothyroidism
– Decreased TRH
– Very rare
Primary Hypothyroidism
• Hashimoto’s Thyroiditis
• Most common class – 95%
• Two common types
– Lymphocytic-plasmacytic destruction
– Idiopathic atrophy
• Rare types
–
–
–
–
–
–
Iodine deficiency
Goitrogen ingestion
Congenital
Neoplasia
Drug toxicity
Surgery or radioactive I131
Secondary Hypothyroidism
• Failure of pituitary thyrotrophs
– 5% of hypothyroid canines
• Thyroid gland hypoplasia or atrophy
• Causes
– Congenital – rare
– Destruction by neoplasia – rare
– Drug thyrotoxicity – most common of the
uncommon
• Glucocorticoids
• hyperadrenocorticism
Cretinism
• Congenital hypothyroidism – rare
• Hallmarks are decreased growth and
delayed mental development
• Puppies relatively normal at birth except
low birth weight
• Causes early puppy death (2-12 weeks)
• Stenotic ear canals and delayed eye
opening
• A goiter can develop
• Disproportionate dwarfism, delayed
dental eruption, retained puppy coat
• Can be caused by iodine deficiency - rare
Cretinism
• Congenital hypothyroidism – rare
• Hallmarks are decreased growth and
delayed
mental development
Mixed breed pup
• Puppies
Before treatment
relatively normal at birth except
low birth weight
• Causes early puppy death (2-12 weeks)
• Stenotic ear canals and delayed eye
opening
• A goiter can develop
• Disproportionate dwarfism, delayed
dental eruption, retained puppy coat
• Can be caused by iodine deficiency - rare
Cretinism
• Congenital hypothyroidism – rare
• Hallmarks are decreased growth and
delayed
mental development
after 20 days treatment
• Puppies relatively normal at birth except
low birth weight
• Causes early puppy death (2-12 weeks)
• Stenotic ear canals and delayed eye
opening
• A goiter can develop
• Disproportionate dwarfism, delayed
dental eruption, retained puppy coat
• Can be caused by iodine deficiency - rare
Cretinism
• Congenital hypothyroidism – rare
• Hallmarks are decreased growth and
delayed mental development
• Puppies relatively normal at birth except
low birth weight
• Causes early puppy death (2-12 weeks)
• Stenotic ear canals and delayed eye
opening
• A goiter can develop
7 month GSD
• Disproportionate
dwarfism, delayed
pup before
dental treatment
eruption, retained puppy coat
• Can be caused by iodine deficiency - rare
Cretinism
• Congenital hypothyroidism – rare
• Hallmarks are decreased growth and
delayed mental development
• Puppies relatively normal at birth except
low birth weight
• Causes early puppy death (2-12 weeks)
• Stenotic ear canals and delayed eye
opening
• A goiter can develop
• Disproportionate dwarfism, delayed
dental eruption, retained puppy coat
• Can be7 month
caused
GSD pup
bybefore
iodine
treatment
deficiency - rare
Cretinism
• Congenital hypothyroidism – rare
• Hallmarks are decreased growth and
delayed mental development
after 1 year therapy
• Puppies relatively normal at birth except
low birth weight
• Causes early puppy death (2-12 weeks)
• Stenotic ear canals and delayed eye
opening
• A goiter can develop
• Disproportionate dwarfism, delayed
dental eruption, retained puppy coat
• Can be caused by iodine deficiency - rare
Clinical Presentation
• Onset over 1-3 years, but can vary
• Four Stages
– Stage I - subclinical thyroiditis
• Positive thyroid autoantibodies (TAb)
– Stage 2 – subclinical hypothyroidism
• increased TSH, positive TAb
• Normal T3, T4
– Stage 3 – clinical hypothyroidism
• Low T3, T4
• Positive TAb, + increased TSH
– Stage 4 – atrophic hypothyroidism
• TAb go negative
• Thyroid autoantibodies may be a
screening test in predisposed breeds
Clinical Presentation
• Symptoms
– General
• Lethargy, mental dullness, weight gain, cold
intolerance
• 15% reduction in energy expenditure
– Dermatologic (60-80%)
• Endocrine alopecia, dry brittle faded coat, “rat
tail,” hyperpigmentation, seborrhea,
pyoderma, otitis, myxedema
– Neuromuscular
• Polyneuropathy, polymyopathy, vestibular
signs, facial/trigeminal paralysis, seizures,
circling/disorientation, myxedema coma,
aggression, altered behavior, laryngeal
paralysis (?)
Clinical Presentation
• Symptoms
– Ocular
• Corneal lipid deposits, KCS, Horner’s
Syndrome
– Cardiovascular
• Bradycardia, arrhythmia, dilated
cardiomyopathy
– Gastrointestinal
• Megaesophagus (?), diarrhea, constipation
– Hematologic
• Anemia, hyperlipidemia
Clinical Presentation
• Symptoms
– Ocular
• Corneal lipid deposits, KCS, Horner’s
Syndrome
– Cardiovascular
• Bradycardia, arrhythmia, dilated
cardiomyopathy
– Gastrointestinal
• Megaesophagus (?), diarrhea, constipation
– Hematologic
• Anemia, hyperlipidemia
Clinical Presentation
• Dermatologic effects can vary
– Hypertrichosis can result in breeds that do
not shed
– Loss of primary hairs can result in “puppy
coat”
– Loss of undercoat can result in coarse coat
with primary hairs only
– Some will show no changes unless clipped
– Will not regrow after clipping
– Tragic expression – myxedema of the face
– Demodicosis can result
– Check thyroid panel for adult onset
Demodex
Clinical Presentation
• Dermatologic effects can vary
– Hypertrichosis can result in breeds that do
not shed
– Loss of primary hairs can result in “puppy
coat”
– Loss of undercoat can result in coarse coat
with primary hairs only
– Some will show no changes unless clipped
– Will not regrow after clipping
– Tragic expression – myxedema of the face
– Demodicosis can result
– Check thyroid panel for adult onset
Demodex
Clinical Presentation
• Dermatologic effects can vary
– Hypertrichosis can result in breeds that do
not shed
– Loss of primary hairs can result in “puppy
coat”
– Loss of undercoat can result in coarse coat
with primary hairs only
– Some will show no changes unless clipped
– Will not regrow after clipping
– Tragic expression – myxedema of the face
– Demodicosis can result
– Check thyroid panel for adult onset
Demodex
Clinical Presentation
• Dermatologic effects can vary
– Hypertrichosis can result in breeds that do
not shed
– Loss of primary hairs can result in “puppy
coat”
– Loss of undercoat can result in coarse coat
with primary hairs only
– Some will show no changes unless clipped
– Will not regrow after clipping
– Tragic expression – myxedema of the face
– Demodicosis can result
– Check thyroid panel for adult onset
Demodex
Clinical Presentation
• Neurologic signs – demyelination and
mucin deposits
– If severe can result in paralysis
– LMN reflexes
– Cranial nerves especially predisposed
– Central and peripheral neuropathies
– Focal or multifocal
– Acute or chronic
– Static or progressive
– Check thyroid panel for mysterious
neurologic disease & behavior problems
Clinical Presentation
• Neurologic signs – demyelination and
mucin deposits
– If severe can result in paralysis
– LMN reflexes
– Cranial nerves especially predisposed
– Central and peripheral neuropathies
– Focal or multifocal
– Acute or chronic
– Static or progressive
– Check thyroid panel for mysterious
neurologic disease & behavior problems
Clinical Presentation
• Neurologic signs – demyelination and
mucin deposits
– If severe can result in paralysis
– LMN reflexes
– Cranial nerves especially predisposed
– Central and peripheral neuropathies
– Focal or multifocal
– Acute or chronic
– Static or progressive
– Check thyroid panel for mysterious
neurologic disease & behavior problems
Clinical Presentation
• Neurologic signs – demyelination and
mucin deposits
– If severe can result in paralysis
– LMN reflexes
– Cranial nerves especially predisposed
– Central and peripheral neuropathies
– Focal or multifocal
– Acute or chronic
– Static or progressive
– Check thyroid panel for mysterious
neurologic disease & behavior problems
Clinical Presentation
• Reproductive failure
– Testicular atrophy and azoospermia
– Prolonged parturition
– Low puppy survival and unthriftiness
– Failure to cycle and prolonged estrus
interval
– Silent heats, false pregnancy, prolonged
estrual bleeding
– Fetal resorption
– Check thyroid panel for failure to
reproduce
Clinical Presentation
• Cardiovascular signs
– Association with atrial fibrillation
– Bradycardia and 1st & 2nd degree AV block
are more common than Afib
– DCM
• Increased LVIDD and LV
• Decreased LVWS and IVS
• Decreased FS
– Hypothyroidism alone rarely results in
CHF
– Check thyroid panel for myocardial
failure
Clinical Presentation
• Check thyroid panel for KCS resistant
to therapy
• Megasophagus (ME) and laryngeal
paralysis (LP) seem to be likely to be
concurrently present with
hypothyroidism
– Cause and effect has not been established
– Little or no response the thyroid
supplementation
• Hypothyroidism associated with
decreased activity of coagulation
factors VIII, IX and von Willebrand’s (?)
Clinical Presentation
• Clues in the blood work
– Normocytic, normochromic anemia
– Leptocytes – target cells
– High cholesterol (75%)
– Hyperlipidemia
– Mild hypercalcemia
– Elevated liver enzymes
– Hyponatremia if myxedema coma
– Check thyroid panel for hyperlipidemia
Thyroid Testing
•
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TSH
TT4
freeT4 (fT4)
fT4 by ED (equilibrium dialysis)
T3
Free T3 (fT3)
rT3
TAb – T4Ab, T3Ab, thyroglobulinAb
Thyroid Testing
Thumb Rules
• T3 is mostly intracellular, so T3 tests
are rarely recommended
• TSH, TT4, fT4 – most common
screening panel for dogs
• TT4, fT4 – most common screening
panel for cats
• Add TAb + fT4 by ED when you
suspect hypothyroidism, but TT4 not
low
Thyroid Testing
TT4
• Lower in dogs (normal 1.0-3.5 mcg/dl)
than in people (normal 4-10 mcg/dl)
• Labs use RIA (radioimmunoassay) or CLIA
(chemiluminescent immunoassay)
• In House – ELISA
– Helpful, but not as accurate
– If in doubt, send sample to outside lab for
confirmation
• Best practice is to spin, freeze
plasma/serum and send on ice in plastic
tube, if assay will not occur within 5 days
Thyroid Testing
TT4
• Hyperlipidemia and hemolysis do not
interfere with TT4 RIA
• Overlap in reference ranges between
euthyroid and hypothyroid
– “borderline” reference range
• Different reference ranges for breeds
– Sight hounds have lower TT4 and fT4
• Greyhound, Italian Greyhound, Whippet
• Saluki, Borzoi, Sloughi, Afghan, Basenji
• Deerhound, Wolfhound, Alaskan sled dogs
Thyroid Testing
TT4 & fT4 – Thumb Rules
• TT4 >2.0 mcg/dl & fT4 >2.0 ng/dl
– hypoT4 very unlikely
• TT4 1.5-2.0 mcg/dl & fT4 1.5-2.0 ng/dl
– hypoT4 unlikely
• TT4 1.0-1.5 mcg/dl & fT4 0.8-1.5 ng/dl
– Anybody’s guess
• TT4 0.5-1.0 mcg/dl & fT4 0.5-0.8 ng/dl
– hypoT4 possible
• TT4 <0.5 mcg/dl & fT4 <0.5 ng/dl
– hypoT4 very likely
Thyroid Testing
fT4
• fT4 by ED is the gold standard T4 assay
– TAb do not interfere
• fT4 by ED is 86-93% accurate
• TT4 is 75-85% accurate
• TT4 assays for humans can be used in
dogs
• fT4 by ED assays for humans can not
be used in dogs
Thyroid Testing
Thyroid Antibodies
• 15% of hypothyroid dogs have them
• Can cause spuriously increased or
decreased TT4
– Depends on the assay
– Falsely increased more common
Thyroid Testing
TSH (Thyrotropin)
• High with hypothyroidism
• Human assays can not be used for dogs
• All commercial assays have poor
sensitivity for canine hypothyroidism
– Many false negatives
– Up to 40% of hypothyroid dogs have normal
TSH
• Specificity is 90%+
• TSH high = likely hypothyroid
Thyroid Testing
Low TSH not clinically significant
• Commercial tests cannot distinguish
between low normal and low values
• Effective reference range goes down
to zero
Thyroid Testing
TSH Response Test
• Distinguishes hypothyroid from
euthyroid sick
• Human rTSH can be used (Thyrogen®)
– Lyophilized 1.1 mg
– Refrigerate for 4 weeks, freeze for 12
weeks
– Expensive $2,500-3,000
– 10-15 tests ($200-250 per test)
• Bovine TSH is sometimes available
– $150-1200
Diagnosis
1. If non-thyroid illness and low TT4
– Treat illness and recheck TT4
2. If signs of hypothyroidism and no
non-thyroid illness
– CBC, panel, TSH, TT4, fT4
– High TSH, low TT4 & fT4 – Eureka!
– High TSH & low fT4
•
•
likely hypoT4, regardless of TT4
Confirm with TAb + fT4ED
– Normal TSH, low TT4 & fT4
•
Likely hypoT4
Diagnosis
2. If signs of hypothyroidism and no
non-thyroid illness
– Normal TSH & fT4, low TT4
•
•
•
consider euthyroid sick
Investigate conditions that mimic
hypothyroidism – other endocrinopathies,
allergies, etc.
Thyroxine trial or TSH response test if no
non-thyroid illness found
– Normal TSH, TT4, fT4
•
Hypothyroidism ruled out
3. If in doubt, do thyroxine trial or TSH
response test
Euthyroid Sick
Presence of non-thyroid illness
aka Non-Thyroid Illness Syndrome (NTIS)
• Low TT4, Normal TSH & fT4
– fT4 can be high in cats with NTIS
• Normal TSH response test
• No response to thyroxine trial
• All thyroid tests can be affected by nonthyroid illness
• Euthyroid sick dogs can have TT4 <0.5
• Hypothyroid dogs almost never have TT4
by ED > 1.5 mcg/dl
Factors Affecting Tests
Most common are
1. Concurrent illness (NTIS)
2. Drugs (especially glucocorticoids)
3. Random fluctuations of thyroid
hormones
Others:
• Age, Breed, Athletic training
• Gender and OHE status
• Environmental and body temperature
• Body Condition & Nutritional Status
Factors Affecting Tests
Concurrent Illness (NTIS, euthyroid sick)
T3 more suppressed than T4
TSH rarely elevated
• Any systemic illness, surgery or trauma
• Severity of illness is proportional to severity
of suppression
• Inadequate calorie intake
• Dermatopathies and osteoarthritis are
unlikely to cause NTIS
• It is nearly impossible to diagnose
hypothyroidism in a significantly ill dog,
unless illness is due *only* to
hypothyroidism
–
TSH stim, thyroid scan or therapeutic trial may
be required
Factors Affecting Tests
Drugs
• Glucocorticoids decrease TT4, fT4, T3,
often into hypothyroid range
– No corresponding increase in TSH
– Topical same effect as internal
– Glucocorticoids withheld 4-8 weeks prior
to thyroid testing
•
Anticonvulsants
– Phenobarbital decreases TT4 and fT4 into
hypothyroid range
•
Also phenytoin, primidone, diazepam
– Bromide could potentially interfere with
iodide uptake by the thyroid
Factors Affecting Tests
Drugs
• Sulfonamide antibiotics
– Truly suppress the thyroid
– Interfere with T4 & T3 synthesis
– TT4 can decrease to hypothyroid range
within 1-2 weeks
– TSH can increase within 2-3 weeks
– Clinical signs can result, chronic therapy can
result in goiter
– Thyroid function returns to normal in 1-12
weeks
– I avoid sulfonamides in hypothyroid dogs
Factors Affecting Tests
Drugs
• NSAIDs
– Decrease TSH, T4 & T3
– Only Etogesic causes clinical signs - KCS
• Tricyclic antidepressants
– Inhibit T4 and T3 synthesis
– Be sure these drugs are not used when
assessing thyroid panel in dogs with
behavior problems
Factors Affecting Tests
Drugs
• Other drugs that decrease T3 & T4
– Amiodarone (T3), propranolol,
dopamine, nitroprusside, furosemide,
heparin
– Androgens
– Imidazoles – methimazole
– Mitotane, propylthiouracil
– Penicillin
– phenothiazines
– Contrast agents – iodide, ipodate (T3)
Factors Affecting Tests
Drugs
• Drugs that increase T3 & T4
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Amiodarone (T4)
Estrogens
5-fluoruacil
Halothane
Insulin
Narcotic analgesics
Contrast agents – ipodate (T4)
Thiazide diuretics
Factors Affecting Tests
Age, Breed, Athletic Training
• Progressive decline of TT4, fT4 & T3 with
age
– Puppies at high end of normal range
– Geriatrics at low end of normal range
– Geriatrics have 30% less than puppies
•
•
•
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Older dogs have higher TSH and blunted
TSH response
Higher TAb in older dogs
Smaller dogs may have higher TT4
Sight hounds & athletes have lower TT4
Factors Affecting Tests
OHE status for females
• Progesterone (diestrus) increases T4
and T3
Environmental and body temperature
• TT4 and fT4 increase in January and
fall in outdoor dogs
Body Condition & Nutritional Status
• Obese dogs have higher T3 and T4
• Fasting >48 hours decreases T3
• Starving dogs can have very low TT4
Treatment
1. Starting dose - 0.01 mg/lb PO BID
– Maximum starting dose 0.8 mg
– Dose is 10x the human dose
– t1/2 of L-thyroxine is 9-14 hours
– BID administration results in less T4
fluctuation
Treatment
2. Recheck 6-8 weeks
– Draw blood 4-6 hours post pill
– TT4 or fT4 should be upper half of
normal to mildly increased (3-6 mcg/dl)
– TSH should be normal (<0.6 ng/ml)
– If TT4 or fT4 are lower half of normal
•
•
If clinical signs controlled and TSH normal,
there is no need to increase
If symptoms not controlled, increase the
dose
– Reduce dose of TT4 >6 mcg/dl
Treatment
3. Adjust dose as needed
– Might need to go as high as 0.02 mg/lb
PO BID
– Higher than that – look for other
problems, or try another brand
•
•
•
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•
Hyperadrenocorticism
Drug therapy
Concurrent illness
Give on an empty stomach to improve
absorption
Make sure taking 4-6 hours post pill
Treatment
4. Once controlled, may be able to
reduce dose to SID
5. Long term monitoring
– Check TT4
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•
•
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2-4 weeks after dosage adjustment
once yearly
signs of thyrotoxicosis
poor response the therapy
– Can also check TSH if initially elevated
– Can use fT4ED if autoantibodies are a
problem in the patient
•
Typically becomes less necessary with time
Treatment
Special Exceptions
• Dilated cardiomyopathy
– Thyroid supplementation increases
myocardial oxygen demand, increases
HR and may reduce filling time
– Starting dose at 25-50% of usual to
prevent decompensation
– Increase dose incrementally as needed
Treatment
Special Exceptions
• Diabetes mellitus
– Hypothyroidism can result in insulin
resistance
– Monitor for hypoglycemia for 1-2 weeks
after introducing or increasing thyroxine
dose
– Reduce insulin as needed
Treatment
Special Exceptions
• Addison’s Disease
– T4 increases cortisol clearance
– Monitor for Addisonian symptoms for 12 weeks after introducing or increasing
thyroxine dose
– Increase or add glucocorticoids as
needed
– If a patient crashes after starting Lthyroxine, run an ACTH Stim test
Treatment
Special Exceptions
• Anticonvulsant, Glucocorticoid
therapy, Severe illness
– Target TT4 in lower half of normal range
Treatment
Treatment Failure
• If treating dermatopathy, consider other
diagnoses
– Atopy
– Other causes of endocrine alopecia
•
If concurrent GI disease (poor
absorption), try T3 supplementation
(liothyronine)
– 4-5 mcg/kg PO TID
– monitor T3 rather than T4
•
Blood just before and 2-4 hours post pill
– May be able to reduce to BID when well
controlled
– Risk of thyrotoxicosis is higher
Treatment
T3/T4 Combination products (1:4)
• Liotrix®, Thyrolar®
• Not recommended for hypothyroid
dogs, because:
– T1/2 and frequency of administration
differ for T3 and T4
– Can result in T3 thyrotoxicosis
– More expensive
Treatment
Thyroid Extracts
• Armour Thyroid®
• Each 60-64 mg tablet = 0.38mg T4,
0.09mg T3 (4:1)
• Challenges:
– Hypersensitivity
– Variability in shelf life and content
Treatment
Myxedema Coma (very rare)
• Early recognition and aggressive
therapy are critical to survival
1. IV L-thyroxine administration
– 0.04-0.05 mg IV BID
– 50% that dose if CHF
2. PO L-thyroxine when stabilized (2448 hours)
3. Correct hypothermia, hypovolemia,
electrolyte disturbances,
hypoventilation
Treatment
Thyrotoxicosis
• Short L-thyroxine t1/2 makes toxicity of
therapy rare
– Can happen with liver or renal failure
•
•
If treatment results in thyrotoxicosis,
reconsider diagnosis
Symptoms:
– Panting, nervousness, anxiety, aggressive
behavior
– Tachycardia, hypertension
– PU-PD, polyphagia with weight loss
– If concurrent cardiac disease - syncope
•
Discontinue for 1-3 days, then resume
lower dose
Prognosis
• Energy improves and mentation
normalizes within 7-10 days of
beginning therapy
• Lipemia and anemia resolve in 2-4
weeks
• Improve over 1-4 months:
– Dermatologic
– Neurologic
– Myocardium
– Reproductive (up to a year)
Prognosis
• Life expectancy if treated is normal for
primary hypothyroidism
• Survival of myxedema coma depends on
early recognition and aggressive
treatment
• Secondary hypothyroidism – guarded to
poor
• Long term survival for cretins is guarded
– Depends on severity of musculoskeletal
changes and age of therapy
– Degenerative joint disease and angular limb
deformities can be severe
Feline Hypothyroidism
• Usually a result of therapy
– Radioactive iodine (I131)
– Bilateral thyroidectomy
– Anti-thyroid drugs
• Clinical signs very rare
– Often transient, resolving within 90 days
– Azotemia most likely
• Spontaneous hypothyroidism almost
unheard of in the cat
• Due to head trauma equally rare
• Cretinism rare, but documented
– Inherited in Abyssinians
Eli
• 2 year old neutered male Great
Pyrenees – Chow mix
• CC – “Sometimes he looks at me like
he doesn’t know me.”
– Also, he laid
down in Lowe’s
the other day
and would not
get up.
Eli
• Exam
– overweight
• Cardiovascular exam
– NSAF
• Neurologic exam
– Cranial nerves, spinal nerves, postural
reflexes normal
• CBC, panel (HW Test current)
– Cholesterol 385 (not fasted)
Eli
• In House TT4, send out
Thyroid panel
– TT4 < 0.5 mcg/dl (undetectable)
– TSH normal, TT4 0.3 mcg/dl, fT4 0.2 ng/dl
• Eventually settled on 1.0 mg PO BID
– Behavior returned to normal
• Died at 8 years of age of adrenal
neoplasia
• The Story of Eli
Summary
– PowerPoint Handout goes behind the
blue tab – Hyperthyroidism, blue sheet,
Hypothyroidism
– Client Handouts
•
•
Hyperlipidemia
Hypothyroidism
– Drug Handouts
•
L-thyroxine
Acknowledgements
J Catharine Scott-Moncrieff. Canine & Feline
Endocrinology, 4th Edition. Ch 3 –
Hypothyroidism.
Raymond P. Bouloy, DVM, Diplomate ABVP
(canine / feline). Cypress Creek Pet Care,
Cedar Park, TX.
2012 Western Veterinary Conference
Roundtable Proceedings. Nutritional
Management of Feline Hyperthyroidism,
Hill’s Prescription Diets.