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DISEASE · CATS

Hyperthyroidism in cats: T4, liver enzymes and the kidneys

Hyperthyroidism is the most common hormonal disease of older cats. An overactive thyroid speeds everything up: the cat eats more but loses weight. Blood tests confirm it with the T4, but they also need careful reading, because the disease raises liver enzymes and can hide kidney disease.

Written by the AI DiagAnimals editorial teamReviewed by the AI DiagAnimals TeamUpdated Our editorial policy

The essentials

Hyperthyroidism affects up to about 10% of cats over 10 years of age. Typical signs are weight loss despite a good appetite, thirst, restlessness and vomiting. It is confirmed by a persistently high total T4, with free T4 or TSH when T4 is borderline. Liver enzymes often rise mildly. Because it lowers creatinine, it can mask kidney disease, which may appear after treatment. Blood pressure is checked at every visit.

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What hyperthyroidism is

It is the most common endocrine disorder of middle-aged and older cats, affecting up to about 10% of cats over 10 years of age in the United States. It is usually caused by benign nodules of the thyroid, often on both sides; only about 2% of cats have a thyroid cancer at diagnosis [1].

Symptoms of hyperthyroidism in cats

  • Weight loss despite a good or increased appetite.
  • Drinking and urinating more.
  • Restlessness, more vocalisation, fast breathing or a fast heart rate.
  • Vomiting, diarrhoea, an unkempt coat and muscle loss along the back.

The typical cat is over 8 years old, active, eats well and loses weight; signs can be subtle when the disease is caught early [1].

How it is diagnosed

TestIn hyperthyroidismWhat to know
Total T4HighA persistently high T4 with typical signs confirms the diagnosis [1].
Free T4 (fT4)HighMore sensitive, but falsely high in about 6% of cats with other illnesses; useful when T4 is borderline [2].
TSHSuppressedLow in 98% of hyperthyroid cats; adds specificity when combined with T4 [3].
ScintigraphyOveractive thyroid tissueImaging used in specialised centres [1].

How to read the T4 line

Here is an example line from an older cat.

An annotated line from a lab report · Illustrative example
AbbreviationResultUnitLaboratory intervalFlag
TT4 Total thyroxine7.8µg/dL0.8–4.7H
  1. Abbreviation TT4, T4 or thyroxine.
  2. Result 7.8 µg/dL in this illustrative line: clearly high.
  3. Unit µg/dL in the United States, nmol/L in Europe (µg/dL × 12.87 = nmol/L): 7.8 µg/dL ≈ 100 nmol/L.
  4. Laboratory interval An illustrative interval; each laboratory sets its own.
  5. Flag H: above the interval. With typical signs, this confirms hyperthyroidism [1].

What else the blood work shows

  • Liver enzymes: hyperthyroid cats often have mild increases in ALT, AST and ALP; the ALP comes mostly from bone [4]. See high liver enzymes in cats.
  • Kidney values: muscle loss lowers creatinine, so hyperthyroidism can hide kidney disease, which may appear once it is treated [1]. See kidney disease in cats.
  • Blood pressure: high in 10–23% of cats at diagnosis, and about a quarter of cats with normal pressure at diagnosis develop high pressure after treatment [5].
  • Fructosamine is lower in hyperthyroid cats, which can mask diabetes [6].

Treatment and monitoring

Treatments include radioactive iodine, anti-thyroid tablets, surgery or an iodine-restricted diet, with overall success rates of 83–99% [1]. Vets recheck T4, kidney values and blood pressure 2 to 4 weeks after starting treatment or changing a dose, then every 4 to 6 months once stable [1]. The AAFP guidelines advise treating all hyperthyroid cats, including those with kidney disease, while avoiding an underactive thyroid [1].

Related articles

Kidney disease in cats

Often unmasked by the treatment of hyperthyroidism.

Read the article

High liver enzymes in cats

Why ALT and ALP rise in hyperthyroid cats.

Read the article

En français : l’hyperthyroïdie du chat.

Questions to ask your veterinarian

  1. Is the T4 clearly high, or should we confirm with free T4 or TSH?
  2. What do the kidney values look like, and how will we watch them during treatment?
  3. What is my cat’s blood pressure?
  4. Which treatment suits my cat best?
  5. When should T4 and the kidney values be rechecked?
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Frequently asked questions

What are the signs of hyperthyroidism in cats?

Weight loss despite a good appetite, thirst, restlessness, vocalisation, vomiting and an unkempt coat, usually in cats over 8 years old [1].

Is it cruel not to treat a cat with hyperthyroidism?

Untreated hyperthyroidism progressively damages the heart and causes weight loss and high blood pressure. The AAFP guidelines recommend treating all hyperthyroid cats [1, 5].

How long does a cat live with hyperthyroidism?

With treatment, median survival of up to about 5 years has been reported in cats without kidney disease [1].

Why did my cat’s kidney values rise after treatment?

Hyperthyroidism lowers creatinine and can hide kidney disease. When the thyroid is controlled, the true kidney function appears [1].

Does hyperthyroidism raise liver enzymes?

Yes, mild increases in ALT and ALP are common and usually improve with treatment [4].

Sources

  1. Carney HC et al. 2016 AAFP Guidelines for the Management of Feline Hyperthyroidism. J Feline Med Surg. 2016;18(5):400-416. doi.org/10.1177/1098612X16643252
  2. Peterson ME et al. Measurement of serum concentrations of free thyroxine, total thyroxine, and total triiodothyronine in cats with hyperthyroidism and cats with nonthyroidal disease. J Am Vet Med Assoc. 2001;218(4):529-536. doi.org/10.2460/javma.2001.218.529
  3. Peterson ME et al. Evaluation of serum thyroid-stimulating hormone concentration as a diagnostic test for hyperthyroidism in cats. J Vet Intern Med. 2015;29(5):1327-1334. doi.org/10.1111/jvim.13585
  4. eClinPath, Cornell University College of Veterinary Medicine. Alanine aminotransferase. eclinpath.com/chemistry/liver/liver-injury/alanine-aminotransferase
  5. Taylor SS et al. ISFM Consensus Guidelines on the Diagnosis and Management of Hypertension in Cats. J Feline Med Surg. 2017;19(3):288-303. doi.org/10.1177/1098612X17693500
  6. eClinPath, Cornell University College of Veterinary Medicine. Fructosamine. eclinpath.com/chemistry/energy-metabolism/fructosamine

Laboratory methods and reference intervals vary. This article explains; it does not diagnose. For a decision about your pet, use the original report and talk to your veterinarian. Our editorial policy.

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