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 ¹³¹iodine-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 ¹³¹sodium 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 (T₄) concentrations, and high cTSH concentrations or abnormal results on a thyrotropin-releasing hormone response test. Samples for measurement of cTSH and T₄ 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.