EFFECT OF DEXAMETHASONE ADMINISTRATION ON SERUM THYROID-HORMONE CONCENTRATIONS IN CLINICALLY NORMAL HORSES

The effect that 5 consecutive days of treatment with dexamethasone (0. 04 mg/kg of body weight, IM, q 24 h) would have on baseline concentrat ions of triiodothyronine (T-3), thyroxine (T-4), reverse T-3 (rT(3)), free T-3 (FT3), and free T-4 (FT4), and on response to thyroid-stimula ting hormone (TSH) administration was determined in 12 clinically norm al horses. Results of TSH response tests indicated that the horses cou ld be placed into 2 groups: in 6 horses (group A), T-4 concentration a fter administration of TSH was less than twice the baseline concentrat ion. Baseline serum concentrations of T-3, T-4, rT(3), FT3, and FT4 we re not significantly different between group-A adn group-B horses. In both groups of horses, serum T-3, T-4, rT(3), and FT4 concentrations w ere significantly increased 6 hours following TSH administration, comp ared with baseline concentrations. Treatment with dexamethasone result ed in significant (P < 0.05) increases in baseline concentrations of r T(3) and FT3 in group-A horses and baseline concentrations of rT(3) in group-B horses. The response to TSH administration following dexameth asone treatment appeared to be blunted with significant (P < 0.05) inc reases only in T-3, T-4, and FT4 concentrations in group-A horses and FT4 concentration in group-B horses. The magnitude of change in serum FT3 concentration in response to TSH administration was significantly less (P = 0.05) following dexamethasone treatment, compared with magni tude of change prior to dexamethasone treatment. Results suggested tha t TSH response testing may not be as valuable as once thought for diag nosing hypothyroidism in horse or for differentiating thyroidal from n onthyroidal illness.