Effect of time of sample collection on serum thyroid-stimulating hormone concentrations in euthyroid and hypothyroid dogs

Objective-To determine effect of time of sample collection on serum thyrotropin (canine thyroid-stimulating hormone [cTSH]) concentrations in euthyroid and hypothyroid dogs. Design-Prospective study. Animals-6 healthy adult euthyroid dogs, 6 adult Beagles with (131)iodine-induced hypothyroidism before and during administration of levothyroxine sodium, and 6 adult dogs with naturally developing hypothyroidism. Procedure-Healthy euthyroid dogs were identified. Hypothyroidism was induced by administration of (131)sodium iodide and confirmed by thyroid-stimulating hormone testing. These dogs then received levothyroxine for 30 days. Naturally developing hypothyroidism was diagnosed on the basis of clinical signs, low serum thyroxine (T-4) concentrations, and high cTSH concentrations or abnormal results on a thyrotropin-releasing hormone response test. Samples for measurement of cTSH and T-4 concentrations were obtained at 2-hour intervals from 8 aM to 8 PM. Results-Mean (+/- SD) serum cTSH concentrations for healthy dogs, dogs with induced hypothyroidism before and during treatment, and dogs with naturally developing hypothyroidism were 0.11 +/- 0.08, 3.31 +/- 1.30, 0.08 +/- 0.07, and 0.55 +/- 0.27 ng/ml, respectively. Diurnal variation in cTSH concentrations was not detected. Clinically important random fluctuations in cTSH concentrations were detected for dogs with naturally developing hypothyroidism. Clinical implications-Sample collection time does not appear to predictably influence cTSH concentrations; however, dogs with naturally developing hypothyroidism may have random fluctuations in cTSH concentrations.