Over-production of thyroid hormone has been a relatively common condition in cats for some years. It is now being diagnosed with greater frequency in older guinea pigs (over 3 years of age) and presents in a similar way. The cause of the over-activity of the thyroid gland can be due to hyperplasia (enlargement of the gland), adenoma (a benign cancer) or carcinoma (a malignant cancer). The relative proportions of these causes in guinea pigs have yet to be determined.
Thyroid hormones are responsible for several body processes, the most notable of which is metabolic rate. Increased thyroid hormone production and activity leads to an increase in metabolism which accounts for the clinical signs. Common clinical signs seen in a guinea pig with hyperthyroidism include:
·Significant weight loss despite a normal or increased appetite.
·Increased activity or skittish behaviour.
·A palpable enlargement of the thyroid gland in the neck, known as a goitre.
Increased heart rate, sometimes accompanied by signs of heart disease e.g. increased respiratory rate and effort.
More rarely seen signs include an increased body temperature, soft faeces, excessive drinking and urinating and hair loss.
Diagnosis can be difficult. Your vet may make a presumptive diagnosis based on the clinical signs. Trial treatment can then be started and response assessed. For a more definitive diagnosis a needle aspiration of the thyroid swelling can be taken or blood sample taken to check for thyroid hormone levels. Blood sampling in guinea pigs is technically challenging and requires the guinea pig to be anaesthetised.
If your guinea pig has an overactive thyroid gland there are three treatment options, medicine, surgical removal of the affected thyroid gland, or the use of radioactive iodine. Medical management is often the treatment of choice as it is non-invasive, and guinea pigs generally respond quite well. A liquid medicine designed for cats is often used. This is usually given twice daily and will need to be given for the rest of the guinea pigs life. Surgery is technically challenging, will only be performed by a small number of vets, requires the guinea pig to be anaesthetised and comes with some possible complications. The use of radioactive iodine is limited to only a handful of referral centres, is costly and requires the patient to be hospitalised for a number of days.
In conclusion if your guinea pig is losing weight despite eating and, usually, defaecating normally an overactive thyroid gland should be considered. Presumptive diagnosis can be based on clinical signs and response to treatment; and medical management often works quite well, halting weight loss and allowing the guinea pig to have a good quality of life.