EFFECT OF STORAGE-CONDITIONS ON CORTISOL, TOTAL THYROXINE, AND FREE-THYROXINE CONCENTRATIONS IN SERUM AND PLASMA OF DOGS

Objective-To determine for dogs stability of cortisol, thyroxine (T-4) , and free thyroxine (fT(4)) in plasma and serum stored in glass or pl astic tubes at -20, 4, 25, and 37 C, Design-Prospective study. Animals -Phase 1, 7 Greyhounds; Phase ii, 6 mixed-breed dogs. Procedure-Phase I: blood was obtained after administration of thyroid-stimulating horm one and adrenocorticotropin. Serum and plasma samples from each dog we re divided into 8 aliquots, 4 in glass and 4 in plastic tubes. A pair of aliquots, 1 in plastic and 1 in glass, were stored at -20, 4, 25, o r 37 C for 5 days and then assayed for hormones. Phase II: blood was o btained without prior stimulation. For fT, determination, serum from e ach dog was placed in plastic or glass tubes, assayed immediately, sto red at -20 C for 5 days, and reassayed, Aliquots from each dog were al so stored for 1 day at 4 or 25 C and then assayed. Samples for cortiso l determination were handled as in phase I. Results-Phase I: there was no effect of tube type (glass vs plastic) on cortisol. Cortisol conce ntrations decreased after storage in serum at 4, 25, and 37 C, and in plasma at 37 C, compared with storage at -20 C. There was no effect of sample type (serum or plasma) on T-4. Thyroxine concentrations increa sed after storage at 37 C in glass, compared with storage at -20 C. Th e fT(4) concentrations were lower in serum than plasma after storage a t -20 C. Concentrations oi fT(4) increased after storage at 37 C in gl ass, compared with storage at -20 C, Phase II: the fT(4) concentration s did not change after storage in any condition. There was no effect o f tube type on cortisol concentrations. Serum cortisol concentrations decreased after storage at 37 C, compared with storage at -20 C. Clini cal Implications-for cortisol, cooling of plasma is not necessary, but serum should be shipped cold. For T-4 and fT(4), serum is sufficient; contained within plastic tubes, samples can be shipped without coolin g if assayed within 5 days.