The goal of thyroidectomy is to remove the abnormal thyroid tissue, leaving the normal adjacent tissue alone. This sounds straightforward, however, 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. Radiotherapy is the treatment of choice for feline hyperthyroidism largely because of its low-risk and low-stress nature (basically a few injections, a few days of boarding in the radiotherapy facility and some temporary home restrictions); however, there are plenty of geographic areas that do not have nearby radiotherapy facilities and surgery is still performed. For pet families opting for surgery, it is important to understand the precautions needed and issues of concern.
Illustration by Wendy Brooks, DVM
- First, a nuclear medicine scan is done prior to surgery. This is important as the scan will provide important information to the surgeon. For example, 20 percent of hyperthyroid cats will have active thyroid tissue in areas besides their thyroid glands. In this situation, surgical removal of all the active tissue will be too complicated and a different therapy should be selected. The scan will identify this situation so that the patient can be spared an ineffective thyroidectomy. In another important situation, 10-30% of cats will only have one diseased thyroid lobe and only that lobe will require removal. It is important that the surgeon knows which lobe should be removed as it is likely not to be visually obvious. The scan will show clearly which lobes of the thyroid gland are diseased and should be removed. The importance of the pre-op scan cannot be over-emphasized.
- Once the need for surgery has been established and the correct thyroid lobes have been identified, the next step is to stabilize the cat for surgery. Most hyperthyroid cats are elderly and have heart disease resulting from their thyroid condition. To set the patient on the road to recovery, the thyroid level is brought into the normal range with two to four weeks of oral medication (usually methimazole - see the oral medication page). Alternatively, certain heart medications (propranolol or other beta-blockers) can be used to compensate for the heart disease associated with hyperthyroidism especially in cats with resting heart rates greater than 220 beats per minute. Any blood pressure, heart disease, electrolyte imbalances (especially low potassium levels), or reduced kidney function must be identified and stabilized prior to surgery. After the cat has recovered from surgery and thyroid levels have normalized, it is important to watch for an exacerbation of renal disease as this may be unmasked by the treatment of hyperthyroidism. Concurrent kidney problems complicates anesthesia and may even preclude the surgery so it is especially important to identify it and control it prior to surgery.
Once the cat has been screened for concurrent problems and thyroid levels have been brought into the normal range, then surgery can be scheduled. Surgical complications include bleeding, damage to local nerves in the throat area, and issues relating to blood pressure and heart disease but, by far, the biggest concern is low blood calcium due to damage of the parathyroid glands that sit adjacent to the thyroid. See below for the advantages and disadvantage of treating hyperthyroidism surgically.
Advantages of Surgical Treatment
- 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. (There may not be facilities nearby for the nuclear medicine scan, however.)
- Approximately 30 percent of hyperthyroid cats have only one thyroid lobe affected. In this situation, the worrisome calcium crisis is not an issue as only one side of the thyroid gland will be removed and the opposite side (and its associated parathyroid glands) will be untouched. In this situation, recovery is generally swift.
Disadvantages and Potential Complications of Surgical Treatment
- 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 six to 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 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 Horner's syndrome. 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 patient’s kidney function be thoroughly evaluated prior to therapy. The effect of restoring normal thyroid function on the patient’s kidneys must be evaluated before beginning permanent resolution of the thyroid disease.
- It takes one to three 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.
- The importance of the pre-operative nuclear medicine scan was mentioned. The chances are that if nuclear medicine is available then radiotherapy is also available. If nuclear medicine scanning is not available, then surgical success will rely on the surgeon's ability to determine visually which thyroid lobe is abnormal or if both of them are. There is commonly asymmetry in bilaterally diseased cats and symmetry in unilaterally diseased cats. The surgeon may not be able to identify the extent of the abnormal tissue correctly.
- Calcium crisis and thyroid storm (see below for details).
Calcium Crisis: The Most Important Potential Complication of Thyroidectomy
This is a serious risk for cats who must have both thyroid glands removed. The blood calcium level must be regulated tightly for normal muscle contraction (including heart muscle) to be possible. This regulation is controlled by the tiny parathyroid glands associated with the thyroid glands. If they are damaged, calcium crisis is a possible risk. There are four parathyroid glands with each lobe of the thyroid gland having one parathyroid embedded within and one sitting on top.
- Signs of hypocalcemia include anxiety, appetite loss, twitching, and facial itch. These signs can progress to seizures and/or heart arrhythmia, hence the importance of monitoring calcium levels after surgery.
- Most hypocalcemia shows up in the 24-96 hours following surgery it can show up up to five days post-operatively. Mild calcium reductions are expected with surgery-associated swelling but if the level drops below 7 mg/dl, seizures are likely and the patient is in danger.
- 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.
- Some surgeons prefer to perform thyroidectomy in stages, removing one thyroid lobe and transplanting the associated parathyroid gland into a local muscle belly to preserve its blood supply. The same procedure is performed on the other thyroid lobe two to three weeks later. The staged procedure reduces the risk of hypocalcemia but does involve two anesthetic procedures on the senior feline patient who is already somewhat debilitated by the hyperthyroid situation. Hypocalcemia is not a concern for cats requiring removal of only one thyroid lobe.
Thyroid Storm: Another Serious Complication of Hyperthyroidsim
"Thyroid storm" refers to the phenomenon where the thyroid gland releases large amounts of thyroid hormone in response to adrenaline and related hormones of anxiety and fear. The result is a life-threatening increase in heart rate, blood pressure, and body temperature The patient develops heart arrhythmias and shock. The hyperthyroid cat is potentially always at risk for this situation but preparation for surgery is special. Anesthetic drugs must be carefully selected and the patient must not be allowed to become frightened or stressed. Since fear and stress commonly begin at home when the cat carrier comes out and the cat is captured, home tranquilizers may be recommended. Ask your veterinarian if you are concerned about your cat's temperament and fear potential.
VIN News Service commentaries are opinion pieces presenting insights, personal experiences and/or perspectives on topical issues by members of the veterinary community. To submit a commentary for consideration, email firstname.lastname@example.org.