Treatment

Treatment of hypothyroidism consists in life­long administration of synthetic levothyrox­ine. Initial treatment dosages varies from 10-22 μg∕kg SID-BID according to the author and the formulation used, with a maximum of 0.8 mg/dog BID.

In most countries, two veterinary prepara­tions are available: Forthyron© (tablets) and Leventa© (liquid preparation). Registered starting dosages (based on optimal body weight) are 20μg∕kg SID and 10μg∕kg BID (Dixon, Reed, and Mooney 2002; Van Dijl et al. 2014; Daminet 2015).

The patient is reevaluated 1-2 months after initiating therapy and dosage is adjusted based on clinical response, and results of serum TT4. Once euthyroidism is achieved, monitoring every 6 months is advised. Lethargy should improve within 1-2 weeks. Hair regrowth will be more gradual and may take up to 1-4 months. With neurological signs the improvement will be gradual and may take months before resolution. When interpreting the result of TT4, time of sam­pling compared to the administration of the medication, and the effect of a meal, should be taken into consideration. First, if blood sample is drawn just before administration of the medication, it is especially the duration of action of the medication that is evaluated (pre-pill test). Most commonly, blood is taken 3-6 hours after the last medication is administered (post-tablet test) and peak con­centrations are measured. In this case, TT4 is expected to be within the reference range (upper half limit), and a TT4 value just above the reference range is accepted. In most patients, follow-up of TSH does not offer a significant advantage over a measurement of T4 solely (Dixon, Reed, and Mooney 2002; Le Traon et al. 2009). Second, administration of a meal does decrease absorption of L-thyroxine. Therefore, it is recommended to administer L-thyroxine 2-3 hours before a meal. For many owners this is not feasible and therefore it is important to follow the daily routine of the owner also on the day of blood sampling, which may mean that the patient is not fasted.

If major clinical improvement is not observed within 3 months despite adequate thyroid hormone supplementation (verified by TT4 serum concentrations), an erroneous diagnosis or presence of unidentified concur­rent disease should be suspected. As the hypo­thalamic-pituitary-thyroid axis has been inhibited by thyroid hormone supplementa­tion, if the practitioner doubts the diagnosis and would like to investigate thyroid function again, treatment has to be ceased for a mini­mum period of 4-8 weeks. With concurrent hypoadrenocorticism, chronic kidney disease, liver failure, or diabetes, the initial dosage should be decreased by 25-50% and gradually increased over a 1-2 months period.