Treatment
At present the only licensed preparation for treatment of canine hypothyroidism
in the UK is Soloxine. This is identical to the naturally occurring hormone.
Most dogs respond well to once daily treatment and since this is more economic,
this is the suggested approach in cases initially. Tablets should be given
in the mornings in order to assist therapeutic monitoring (see below).
Dosage
The starting dose of Soloxine is 0.02mg/kg given once daily. This should
then be increased or decreased based on the clinical and biochemical response.
Dogs should be allowed two weeks on each new dose before making a judgement
on its effect. The magnitude of dose adjustment is fairly empirical and
often guided by the next available tablet size. Dogs with GI malabsorptive
disease may require higher doses. Some texts recommend introducing therapy
gradually in patients with concurrent illness, based on the human therapeutic
protocol, but there is little evidence to support this approach in dogs
at present. However, it is recommended that dogs with concurrent hypoadrenocorticism
should be monitored carefully during this initial period in case of an increased
physiological glucocorticoid requirement. .
Clinical Monitoring
The response to adequate treatment in a correctly diagnosed hypothyroid
dog should be dramatic. Anything other than this should prompt immediate
consideration of either sub-optimal therapy or a misdiagnosis. Speed of
clinical improvement depends on the system involved: mental demeanour usually
improves within 3-10 days. Hair coat regrowth typically takes 10-12 weeks
to be obviously improved. Note a worsening of hair loss is common often
between weeks two and six of treatment but is not a cause for concern. Weight
loss is clearly progressive but a reasonable expectation is 10% body weight
reduction over 3 months.
Laboratory Monitoring
Combined total T4 and cTSH measurement is routinely recommended.
Circulating Total T4 should be measured 7-10 days after
starting therapy or after changing the dose. Circulating concentrations
of the hormone peak approximately six hours post-pill, then decline. Therefore
monitoring samples must be collected six hours post pill to allow reliable
interpretation. Peak values of approximately 60nmol/L are considered IDEAL
(note this is above the "normal" reference range). Peak
circulating total T4 values less than 35nmol/L suggest INADEQUATE therapy.
Values greater than 100nmol/L probably warrant a dose REDUCTION.
Concurrent circulating cTSH estimation is also recommended
to assist in long term monitoring.
In well treated dogs, cTSH usually returns to the lower end of the reference
range. However, the expected "suppression" of cTSH does not in
itself confirm the adequacy of therapy. Values that remain in the "hypothyroid"
range certainly indicate inadequate treatment.
Routine biochemical and haematological improvements are
expected in well treated hypothyroid dogs. These improvements are progressive
and should occur broadly in line with the clinical changes. |