SURGICAL TREATMENT FOR FELINE HYPERTHYROIDISM
The goal here is to remove the abnormal thyroid tissue, leaving the normal adjacent tissue alone.
Considering that the average hyperthyroid cat is a geriatric patient with special potential for high blood pressure and heart disease, quite a bit of patient preparation is necessary to reduce anesthetic risk.
- First, the patientís excess thyroid level is brought into the normal range with 2-4 weeks of oral medication (usually methimazole - see oral medication page). During the last 1-3 weeks of this period, special heart medication (propranolol or other beta-blocker) is often used to compensate for the heart disease associated with hyperthyroidism especially in cats with resting heart rates greater than 220 beats per minute. After thyroid levels have normalized, it is important to watch for an exacerbation of renal disease as may be unmasked by the treatment of hyperthyroidism. Concurrent kidney problems complicates anesthesia and may even preclude the surgery.
- Ideally a nuclear medicine scan (see radiotherapy page ) is done prior to surgery to determine for sure which thyroid lobe should be removed or if both should be removed. If this is not feasible, the surgeon will have to make a decision based on the visual appearance of the glands at the time of surgery.
In 30% of cases one gland is obviously abnormal and one gland is obviously normal (or even atrophied). The decision about what to remove is easy and the risk of calcium crisis (see below) is not significant.
In 70% of cases, both glands are abnormal and must be removed. In 15% of these cats, though, the enlargement is not symmetrical and, unless a nuclear medicine scan is done prior to surgery, the surgeon may be fooled into leaving an abnormal gland behind. Alternatively, if both glands are removed, there is potential to damage the tiny but very important parathyroid glands which sit atop the thyroid glands.
- Treatment is generally permanent after recovery. Unless complications arise, no further treatment is needed.
- No special facilities are required for this treatment (as they are for radiotherapy) so it is likely that someone who performs this surgery is located conveniently to any owner of a hyperthyroid cat.
- Performing surgery and general anesthesia on a geriatric patient with potential heart disease has inherent risk.
- If abnormal thyroid tissue is left behind, hyperthyroidism is likely to recur within 6-24 months (note: even if normal thyroid tissue is left behind, it is possible for the normal tissue to develop abnormal changes and for the condition to begin afresh.). Continued monitoring of T4 (typically twice a year) is frequently recommended.
- The monitoring before and after surgery plus the surgery itself is relatively expensive.
- The laryngeal nerve is located near the thyroid gland. If it is damaged during surgery, the cat can experience a voice change. This change may be permanent.
- The sympathetic trunk (another neurologic tissue) is located near the thyroid gland. If it is damaged during surgery, the eye on that side may develop what is called ďHornerís syndrome.Ē This eye will squint and draw back into the socket, the pupil will constrict and the third eyelid will come up. This syndrome may be permanent.
- Because this treatment represents a permanent cure, it is important that the patientís kidney function be thoroughly evaluated prior to therapy. The effect of restoring normal thyroid function on the patientís kidneyís must be evaluated before permanent resolution of the thyroid disease is effected.
- It takes 1-3 months for thyroid blood levels to stabilize following surgery. Some cats become Hypo thyroid after surgery and must take thyroid supplementation tablets either temporarily or permanently.
This is a very serious risk for cats who must have both thyroid glands removed. The blood calcium level must be regulated very tightly for normal muscle contraction (including heart muscle) to be possible. This regulation is controlled by the tiny parathyroid glands adjacent to the thyroid glands. If they are damaged, calcium crisis is a possible risk.
- Signs of hypcalcemia include anxiety, appetite loss, twitching, and facial itch. These signs can progress to seizures hence the importance of monitoring calcium levels.
- Blood calcium should be monitored daily for 4-7 days following surgery. A mild reduction is considered normal but should the level drop below 7 mg/dl, the veterinarian must be prepared for seizures.
- Calcium crisis is treated with calcium supplementation and Vitamin D but, as there are several forms of both these products, it is important that you use only products recommended by your veterinarian.
DO NOT ATTEMPT TO TREAT THIS YOURSELF WITH A VITAMIN D SUPPLEMENT FROM THE DRUG OR NUTRITION STORE!
- Supplementation may be needed for only a few days or for life depending on the damage to the parathyroids.
Page last updated: 12/2/07
Page last reviewed: 9/5/2013