Just Ask the Expert: Should thyroid function be monitored in hypothyroid dogs given corticosteroids?

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Some of my hypothyroid canine patients also intermittently receive corticosteroids at anti-inflammatory doses to treat flare-ups of allergic dermatitis. Does the corticosteroid therapy affect thyroid hormone concentrations and interfere with testing—either for the initial diagnosis or for therapeutic monitoring purposes? Are thyroxine supplementation dosage adjustments needed during corticosteroid therapy?

A. Animals receiving anti-inflammatory or immunosuppressive doses of prednisone or prednisolone can have altered thyroid function test results, especially if they have been receiving the corticosteroids for more than two weeks. In general, I would prefer to see animals off of all forms of corticosteroids for at least four to six weeks before trying to evaluate thyroid function.

David S. Bruyette, DVM, DACVIM

In animals receiving thyroid hormone supplementation that subsequently begin to receive corticosteroid therapy, we generally do not perform laboratory tests to evaluate thyroid function until the corticosteroids have been removed. However, a recent paper looked at the effect of short-term anti-inflammatory doses of prednisone in dogs with naturally occurring hypothyroidism.1

Eight adult spontaneously hypothyroid dogs receiving supplementation with thyroxine were given prednisone (1 mg/kg orally) daily for seven days and then on alternate days for 14 days.1 Serum total thyroxine (T4), free T4, and thyroid-stimulating hormone (TSH) concentrations were measured on days 7, 21, and 28 and compared with baseline data. In this group of dogs, total T4 concentrations were significantly decreased after seven days of anti-inflammatory prednisone but were not significantly altered from baseline on days 21 or 28. Free T4 and TSH concentrations were not significantly altered from baseline at any point during the study. Two dogs had decreased total T4 concentrations on day 7, which may have resulted in an alteration in thyroid supplementation.

Results showed that administration of prednisone at a dosage of 1 mg/kg given orally once daily for seven days decreased total T4 concentrations, while free T4 concentrations were unchanged,1 suggesting that free T4 concentrations may be less affected by daily prednisone administration. Anti-inflammatory doses of prednisone administered every other day did not interfere with thyroid hormone monitoring. So, at least with short-term administration of a single daily anti-inflammatory dose of prednisone, thyroid function may be evaluated by looking at free T4 or TSH concentrations. However, these results cannot be generalized to patients taking prednisone for longer periods or at higher doses.

REFERENCE

1. O'Neill SH, Frank LA, Reynolds LM. Effect of an anti-inflammatory dose of prednisone on thyroid hormone monitoring in hypothyroid dogs. Vet Dermatol 2011;22(2):202-205.

David S. Bruyette, DVM, DACVIM VCA

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