The thyroid gland and hormones

TSH

TSH concentration is usually elevated in patients with hypothyroidism due to the absence of negative feedback.

TSH is a species-specific protein, so appropriate tests must be used to obtain an accurate result. In euthyroid sick syndrome, TSH may be elevated. In approximately 35% of dogs suffering from hypothyroidism, TSH concentration may be within the physiological range, so in these cases combining TSH and FT4 values is recommended in order to increase testing sensitivity.

Total T4

Concentration is usually reduced in patients suffering from hypothyroidism, or is close to the lower limit of the reference range.

Because T4 concentration varies throughout the day, a diagnosis of hypothyroidism should not be based on a single value below the lower limit of expected values alone. In more than half of healthy dogs and cats, the total T4 value may be lower than expected.

There are a large number of reasons why T4 may be low that are unrelated to thyroid disorders: use of medications (glucocorticoids, trimethoprim-sulpha, phenobarbitone, clomipramine, salicylates), conditions such as pregnancy, oestrus, obesity, hyperadrenocorticism, hypoadrenocorticism, infections, renal insufficiency and hepatic insufficiency, diabetes mellitus. Patients in whom total T4 concentration is greater than 20 nmol/L are most likely not suffering from hypothyroidism.

Euthyroid sick syndrome is a condition in which concurrent illness leads to reduced thyroid metabolism and decreased circulating hormone concentrations. These patients are euthyroid at the cellular level and do not have clinical hypothyroidism. Because of this syndrome, a detailed clinical examination is essential.

FT4

Free T4 is the metabolically active form of T4. Determination of FT4 concentration gives precise information about the patient's thyroid status. The value of FT4 also varies throughout the day, and in some hypothyroid patients values may be close to the lower limit of expected values.

T3

This hormone is found predominantly intracellularly, so the circulating concentration of T3 is relatively low. It is used as additional confirmation of a low T4 concentration, in order to confirm a diagnosis of hypothyroidism with greater certainty.

SDMA

Hypothyroidism has been associated with reduced glomerular filtration in dogs and cats in several studies. SDMA concentration was significantly elevated in the group of hypothyroid dogs.

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Understanding the interpretation of laboratory findings is a continuous process. With ongoing education and experience, veterinarians can improve their interpretation skills, thereby enhancing the quality of care provided to their patients.

Collaboration with the expert team of a specialist laboratory is also key, as their knowledge and expertise can provide additional guidance which is often essential for a complete understanding of findings.

We address this topic in our workshops. Contact us for more information.

DVM, PhD, MS, Strahinja Medić