Canine hypothyroidism: diagnostic and therapeutic aspects

Hypothyroidism is one of the most common endocrinopathies in dogs. Clinical signs and haematological and biochemical parameters lead to a first tentative diagnosis. The confirmation of this diagnosis using specific hormone tests can, however, be challenging in individual cases. The total serum 14 concentration has a very good sensitivity but as this parameter is affected by various factors (other diseases, medication and dog breed), a low T4 alone cannot be used for confirmation. Additionally, up to 40% of hypothyroid dogs have normal thyroid stimulating hormone (TSH) concentrations, meaning that only an increased TSH is of diagnostic significance. The gold standard for the diagnosis of hypothyroidism in dogs has been for many years the TSH-stimulation test. In this test, recombinant human TSH is given intravenously and the circulating 14 concentration is measured both before and 6 hours after the administration. The degree of increase in T4 is used to determine the functional reserve capacity of the thyroid gland, thereby allowing its hormonal activity to be assessed. The therapy for this condition is the daily oral administration of synthetic thyroxine. Several synthetic thyroxine products approved for use in dogs are available. According to the manufacturers, the dosage recommendations vary between 10 mu g/kg and 20 mu g/kg BID. A liquid formulation is available which is given SID, although in individual cases a change to BID may be necessary. It is important not to assume treatment failure until a maximum dosage has been used for an adequate period of time. In addition, it is also important that the oral administration of thyroxine is given in a standardised manner - i.e. always after food or on an empty stomach - as the bioavailability of thyroxine is greater in the latter situation, meaning that a lower dosage may be needed.