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What is hyperthyroidism ?
Hyperthyroidism results from overactive tissue within the thyroid gland producing an excess of thyroid hormones.
In cats it is caused by benign tumors of the thyroid gland approximately 95% of the time. The remaining 5% or so of affected cats have thyroid cancer. The abnormal tissue functions independently of the normal thyroid regulatory mechanisms, and secretes too much thyroid hormone. The excessive hormone production causes many serious side-effects, such as weight loss, nervousness, liver and heart problems, and high blood pressure. Kidney damage usually occurs. Almost all cats have normal thyroid tissue that is dormant because of the over-activity of the tumors. This normal tissue reactivates after radioiodine treatment, and the cat regains its normal thyroid function.
How is it diagnosed ?
The diagnosis of feline hyperthyroidism is routinely made based on the measurement of elevated circulating levels of the thyroid hormones thyroxine (T4) and triiodothyronine (T3). However thyroid hormone concentrations in cats with hyperthyroidism are subject to significant variation with age (the older the lower), concurrent illness (artificially lower values) and during the day. As a result, the diagnosis of hyperthyroidism may be difficult in some cats, and a thyroid scan may be required for definitive diagnosis.
Thyroid scan imaging is most commonly used in veterinary medicine for evaluating cats with known or suspected hyperthyroidism. In normal cats, the thyroid gland appears on scintigrams as two well-defined focal (ovoid) areas of radionuclide accumulation in the cranial to middle cervical region. The two thyroid lobes are symmetrical and located side by side. Activity in the normal thyroid closely approximates activity in the salivary glands with an expected ratio of 1:1
A thyroid scan is a critical part of your cat's diagnosis and treatment plan because it provides information on
1. Functional status of both thyroid glands, (e.g., hyperthyroid, euthyroid, hypothyroid).
2. Location (Left or right) of hyperfunctioning thyroid tissue. This is especially important if planning surgical excision..
3. Size and extent (unilateral, bilateral or ectopic) of the thyroid lesion responsible for hyperthyroidis.
4. Qualitative assessment of functional status of the thyroid gland(s) lesion
5. Likelihood of hyperfunctioning tissue being malignant
How can it be treated ?
Permanent (curative) modalities include radioiodine therapy or surgical extirpation of abnormal thyroid tissue. Reversible treatment involves long term medical management with methimazole (Tapazole®), or iodinated contrast agents (calcium ipodate). These medical treatments are considered reversible, in that they can be discontinued or dosages lowered in patients that become azotemic when their hyperthyroid state is corrected. However medical treatment do not treat the cause but the clinical signs and do prevent progression of the disease.
Radioactive iodine therapy is considered the optimum treatment for feline hyperthyroidism. It involves a single non-stressful procedure that is without associated morbidity or mortality.
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