It is the opposite of hyperthyroidism with an under-active thyroid. Mostly occurring in dogs this is an insidious disease with subtle changes often over long periods really mimicking ageing. Slowing down of activity, tendency to put on weight, loss of hair. There is sometimes atrophy of facial muscles and a rat like tail (loss of fur) may develop. These are the kind of changes you may wish to bring to our attention at a visit to the surgery. We will be on the look out at booster / annual check up time. Diagnosis is by blood test. Treatment is highly successful.