What do T4 and TSH measure?

Total thyroxine (T4) is the main thyroid hormone, and thyroid-stimulating hormone (TSH) is the pituitary hormone that regulates it. Together they assess thyroid function: high T4 with suppressed TSH indicates hyperthyroidism, while low T4 suggests hypothyroidism.

What is the difference between hyper- and hypothyroidism in cats?

Hyperthyroidism (overactive thyroid) is common in older cats and causes weight loss, increased appetite, and hyperactivity. Hypothyroidism (underactive thyroid) is rare in cats and more common in dogs, causing lethargy and weight gain.

How is feline thyroid disease diagnosed?

Thyroid disease is diagnosed by measuring T4 (and TSH) in serum or plasma, ideally with quantitative assays that provide precise concentrations and age-appropriate reference ranges. Serial monitoring helps titrate therapy once treatment begins.

Feline Thyroid Testing: T4 and TSH Explained

Thyroid disease is common in companion animals — but it presents very differently in cats than in dogs. Cats typically develop hyperthyroidism (an overactive thyroid), while dogs typically develop hypothyroidism (an underactive thyroid). Understanding the relevant biomarkers — thyroxine (T4) and thyroid-stimulating hormone (TSH) — is key to accurate diagnosis. This guide explains thyroid testing.

The Thyroid Axis

The thyroid gland produces thyroxine (T4), the primary thyroid hormone, which regulates metabolism throughout the body. T4 production is controlled by thyroid-stimulating hormone (TSH), released by the pituitary gland.

This forms a feedback loop:

Low T4 → pituitary releases more TSH → stimulates thyroid.
High T4 → pituitary releases less TSH → suppresses thyroid.

Measuring both T4 and TSH reveals where in this axis the problem lies.

Hyperthyroidism in Cats

The Disease

Hyperthyroidism — an overactive thyroid, usually caused by a benign thyroid tumour — is the most common endocrine disease of older cats. Excess T4 accelerates metabolism.

Signs: weight loss despite a ravenous appetite, hyperactivity, vomiting, increased thirst and urination, unkempt coat, and (in advanced cases) heart problems.

Testing

Total T4 (fTT4) — the primary screening test. Elevated T4 is the hallmark of hyperthyroidism.
TSH — usually low in hyperthyroidism (the pituitary is suppressed).

Interpretation

Elevated T4 — strongly suggestive of hyperthyroidism.
Borderline (“grey zone”) T4 — some hyperthyroid cats have only mildly elevated T4, especially with concurrent illness. Repeat testing or additional diagnostics may be needed.

Related product: Feline T4 Test

Hypothyroidism in Dogs

The Disease

Hypothyroidism — an underactive thyroid, usually from immune-mediated thyroid destruction — is common in middle-aged dogs. Low T4 slows metabolism.

Signs: weight gain, lethargy, hair loss, skin problems, cold intolerance, and behavioural changes.

Testing

Total T4 (cTT4) — the initial screening test. Low T4 suggests hypothyroidism, but can also occur with other illnesses (“euthyroid sick syndrome”).
TSH — elevated in true hypothyroidism (the pituitary is trying to stimulate a failing thyroid). The combination of low T4 + high TSH is strongly diagnostic.

Interpretation

Low T4 + high TSH — confirms hypothyroidism.
Low T4 + normal TSH — inconclusive; consider other illness or repeat testing.
Normal T4 — rules out hypothyroidism in most cases.

Related products: Canine T4 Test · Canine TSH Test

Why Combined T4 + TSH Testing Matters

Measuring both T4 and TSH dramatically improves diagnostic accuracy:

In hypothyroidism, the pattern of low T4 + high TSH is highly specific.
In hyperthyroidism, high T4 + low TSH confirms the diagnosis.
A single marker can be misleading; the combination clarifies.

Quantitative Testing and Monitoring

Thyroid disease is managed over time, making quantitative testing essential:

Dose titration — after starting thyroid medication, T4 is measured to adjust the dose.
Monitoring response — ensuring T4 returns to the normal range.
Detecting recurrence — in cats after radioactive iodine or medication.

Serial quantitative T4 measurement is the backbone of thyroid disease management.

In the Bigger Picture

Thyroid testing is part of the broader biomarker toolkit. For a full overview, see Veterinary Biomarker Monitoring.

Conclusion

Thyroid disease is common and highly treatable — when diagnosed accurately. By understanding the T4–TSH axis and using combined, quantitative testing, veterinarians can diagnose thyroid disease confidently and manage it effectively over the long term.