|Iatrogenic hypothyroidism in a cat. Note the matted hair coat.|
After successful surgical thyroidectomy in a cat with hyperthyroidism, the serum thyroid hormone concentrations (both T4 and T3) should fall to low-normal or low concentrations by 24 hours postoperatively (8-10). I recommend checking a serum T4 concentration before the cat is discharged from the hospital (within 1-2 days of surgery) to ensure that the procedure has been successful in removing all adenomatous thyroid tissue.
If the serum T4 concentration remains high or has only fallen into the high-normal range, it is very likely that remaining adenomatous tissue remains and that the cat will require additional treatment (I'll be covering persistent hyperthyroidism in my next post). If, on the other hand, the serum T4 value is subnormal, treatment for hypothyroidism must be considered.
Hypothyroidism After Unilateral Thyroidectomy
In cats that have unilateral thyroid disease, only one thyroid lobe is generally removed (i.e., unilateral thyroidectomy is performed). Because the remaining "normal" thyroid lobe in these cats has been suppressed and is not functioning normally, serum thyroid hormone concentrations are expected to fall to subnormal levels for 1 to 2 months. This transient hypothyroid state is followed by a return to euthyroidism by 3 months postoperatively, as the remaining thyroid lobe recovers and starts to function once again (8).
After treatment of a hyperthyroid cat with unilateral thyroidectomy, thyroid hormone supplementation is not generally recommended during this period of transient hypothyroidism. The main reason for this recommendation is that thyroid hormone replacement will postpone —and may even prevent— full recovery of normal thyroid function.
The major exception to this rule pertains to cats that have or develop concurrent kidney disease. It is now clear that hypothyroidism (even transient or temporary) can lower renal blood flow and the glomerular filtration rate (GFR), which can lead to worsening of concurrent chronic renal disease (11-14). Treating the hypothyroidism can raise the renal blood flow and GFR to an acceptable level, thus helping to protect kidney function in these cats (15-17).
Hypothyroidism After Bilateral Thyroidectomy
Almost all cats that undergo bilateral or "total" thyroidectomy will become hypothyroid and will benefit from thyroid hormone replacement therapy. Remember that after one performs a total thyroidectomy and removes both adenomatous thyroid lobes, we expect to find undetectable thyroid hormone values. If the serum T4 concentration remains high or only falls to the reference range limits, it is very likely that remaining adenomatous tissue remains and that the cat will require additional treatment (7).
Initial thyroid hormone replacement dose
After bilateral thyroidectomy has been performed, L-thyroxine or L-T4 (0.1 mg, once or twice daily) should be started as soon as we document that the postoperative serum T4 concentration is low to undetectable (8-10). While the use of divided dosing will result in less fluctuation of the circulating T4 concentrations compared to administration of the same total dose as a single daily bolus, the biological action of thyroid hormones (within the tissues and cells) far exceeds that of their serum half-life. This explains why many cats will do well on once-daily L-T4 supplementation.
Either L-T4 pills (e.g, Soloxine, Virbac; Thyro-Tabs, Vetamix) or liquid suspension (e.g., Leventa solution, Merke Animal Health) can be used successfully in cats. When thyroid hormone supplementation is given to cats, the dose should be given at the same time(s) each day.
The supplement can be given either with food or on an empty stomach, but one should be consistent in how it is dosed to avoid marked fluctuations in the absorption of L-T4. Absorption of the L-T4 is likely better when administered on an empty stomach, as has been reported in both humans and dogs (18,19). However, studies comparing absorption of L-T4 in the fed vs. fasting state have not yet been reported in cats. Nevertheless, we can expect that a higher daily dose of L-T4 might be needed if the thyroid hormone supplement is given at the time of feeding (e.g., if the medication is placed in the food).
Monitoring L-T4 supplementation
The ideal replacement dosage is based on the results of a serum thyroid panel, which includes at minimum the determination of serum T4 and TSH concentrations (20-22). This serum thyroid panel is collected 4 hours after the cat's morning dose of L-T4 is administered (23). If serum T4 is low to low-normal and serum TSH concentration is high, the dose of L-T4 should be increased or given twice daily, or both. If the serum T4 is high-normal to high, especially if the cat is showing signs of hyperthyroidism (i.e., weight loss despite a good appetite), the dose of L-T4 should be reduced.
Once the proper daily replacement dose is determined for the individual cat, the T4 supplementation can be safely continued indefinitely. However, in some cats, the low serum concentrations of T4 and T3 may spontaneously increase into the normal reference range after a few weeks to months (6,8,24,25). Small pieces of adenomatous thyroid tissue left attached to the thyroid capsule (in the area of the parathyroid gland) can regrow enough to secrete normal amounts of thyroid hormone. Thyroid hormone administration can then be discontinued.
To evaluate whether or not L-T4 replacement therapy can be discontinued, we must stop the thyroid supplement for at least 2 days and repeat a serum thyroid hormone panel. If normal values are maintained after being off L-T4 for 48 hours, the thyroid hormone supplementation can be discontinued.
Monitoring Cats After Thyroidectomy
In all hyperthyroid cats treated with thyroidectomy, thyroid function testing should be monitored at 6- to 12-month intervals for the rest of the cat's life. In some cats, relapse of hyperthyroidism can develop, especially after many months. Such hyperthyroid cats can be difficult to manage, especially is reoperation is contemplated.
In my next post, I'll be covering both persistent and recurrent hyperthyroidism and how to diagnose and treat these difficult cases.
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