Can Thyroid Function be Monitored in Hypothyroid Dogs Treated with Steroids?

Some of my hypothyroid dogs 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?

My Response:

Glucocorticoids are known to affect serum thyroid hormone concentrations in dogs (1-4). Dogs 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 2 weeks. In general, I would prefer to see dogs off of all forms of corticosteroids for at least 4-to 6-weeks before trying to evaluate thyroid function.

In dogs 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, one paper in 2011 by O'Neill et al did study the effect of short-term anti-inflammatory doses of prednisone in dogs with naturally occurring hypothyroidism (5).

In that report, 8 dogs with spontaneous hypothyroidism already being treated with levothyroxine (L-T4) were given prednisone (1 mg/kg orally) daily for 7 days and then on alternate days for 14 days (5). 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. Results showed that total T4 concentrations were significantly decreased after 7 days of an anti-inflammatory dose of prednisone, but T4 values were not significantly altered from baseline on days 21 or 28 while on every other day dosing. Free T4 and TSH concentrations were not significantly altered from baseline at any point during the study.

My Bottom Line

Based on the results of the O'Neill study (5) administration of prednisone at a dosage of 1 mg/kg given orally once daily for 7 days decreased total T4 concentrations, while free T4 concentrations were unchanged. This suggests that free T4 concentrations may be less affected by daily prednisone administration. Anti-inflammatory doses of prednisone, when administered every other day, did not interfere with thyroid hormone monitoring.

These results also agree with two previous studies, which showed that anti-inflammatory prednisone did not affect serum total T4 concentrations in thyroid-supplemented, thyroidectomized dogs (3,6).

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 dogs taking prednisone for longer periods or at higher immunosuppressive doses (2-4 mg/kg/day).

References

1. Woltz HH, Thompson FN, Kemppainen RJ, et al. Effect of prednisone on thyroid gland morphology and plasma thyroxine and triiodothyronine concentrations in the dog. Am J Vet Res 1983;44:2000-2003. 
2. Torres SM, McKeever PJ, Johnston SD. Effect of oral administration of prednisolone on thyroid function in dogs. Am J Vet Res 1991;52:416-421. 
3. Moore GE, Ferguson DC, Hoenig M. Effects of oral administration of anti-inflammatory doses of prednisone on thyroid hormone response to thyrotropin-releasing hormone and thyrotropin in clinically normal dogs. Am J Vet Res 1993;54:130-135. 
4. Daminet S, Paradis M, Refsal KR, et al. Short-term influence of prednisone and phenobarbital on thyroid function in euthyroid dogs. Can Vet J 1999;40:411-415. 
5. 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;22:202-205.
6. Kaptein EM, Moore GE, Ferguson DC et al. Effects of prednisone on thyroxine and 3,5,3’-triiodothyronine metabolism in normal dogs. Endocrinology 1992;130:1669–1679.