How is feline hyperthyroidism diagnosed?
Diagnosis of feline hyperthyroidism requires provision of evidence of persistently elevated thyroid hormone concentrations alongside one or more of the typical clinical signs. To achieve this, signalment and history should be reviewed, physical examination conducted, and the results of blood tests reviewed
What symptoms do cats with hyperthyroidism display?
Given thyroid hormones affect many body systems, there is a wide range of clinical signs associated with feline hyperthyroidism. However the typical signs displayed include (Carney, 2016):
A goitre may also be palpable upon physical examination. As the size of the thyroid gland increases, so too does the likelihood of feline hyperthyroidism being present (Boretti et al, 2009)
If diagnosis requires measurement of thyroid hormones –
why undertake further biochemical testing?
Routine biochemistry helps to find non-specific changes which can support a diagnosis of feline hyperthyroidism. Equally, it can help to rule out concurrent diseases which can present in a similar way.
Hyperthyroid cats usually have elevations in the liver enzymes alanine aminotransferase (ALT) and alkaline phosphatase (ALKP). It is important to note that the rise should be proportional to the increase in total T4. If the increase in ALT and/or ALKP is significantly greater than the increase in total T4, or if GGT is elevated, further tests of liver function may be required.
Chronic kidney disease (CKD) and diabetes mellitus (DM) can result in similar clinical signs to those of hyperthyroidism. Also, the presence of CKD or DM alongside hyperthyroidism can have implications for treatment of the hyperthyroidism.
Therefore, it is essential to look for these concurrent diseases at the time of the initial investigation.
Which endocrine specific tests are used most commonly in the diagnosis of feline hyperthyroidism?
1. Total T4
Total T4 should be used as the first screening test in the diagnosis of hyperthyroidism. It is cost-effective and establishes a baseline level prior to treatment. A serum Total T4 concentration >60 nmol/l is diagnostic of hyperthyroidism, though it is important to note that reference intervals may differ between individual laboratories.
Measurement of an elevated serum Total T4 will confirm approximately 90% of cases of hyperthyroidism. However, this means the Total T4 can be within the reference interval in approximately 10% of hyperthyroid cats. This is most often seen in early or mildly affected cases or cats with concurrent non-thyroidal illness (NTI), for example CKD.
2. Free T4
If the diagnosis has not been confirmed with a Total T4 test, a Free T4 test could be performed. The Free T4 test only measures the fraction of T4 that is metabolically active and not bound to carrier proteins. Levels of Free T4 are less affected by non-thyroidal illness. Measuring Free T4 involves a more complex technique called equilibrium dialysis, so this test is normally more expensive.
Some non-hyperthyroid cats can have a high Free T4, so it is important to measure Free T4 in conjunction with Total T4 (ideally in the same blood sample) to enhance the diagnostic specificity. For example, up to 20% of sick euthyroid cats have elevated Free T4 concentrations. These cats normally have a corresponding Total T4 in the lower half of, or below, the reference interval.
Measurement of Free T4 alone could yield a false positive diagnosis of hyperthyroidism. Generally speaking, if you have compatible clinical signs, a Total T4 in the upper half of the reference interval and a Free T4 above the reference interval then this is indicative of hyperthyroidism. For further information consult the diagnosis flowchart.
