Development and Validation of a Radioimmunoassay for Thyrotropin in Cattle

Abstract

Abstract. In mammals, thyrotropin, or thyroid-stimulating hormone (TSH), assay is used for the diagnosis of primary hypothyroidism. Hypothyroidism is the most common type of thyroid disorder in cattle. The aim of this study was to develop and validate, under physiologic and pathologic conditions, a radioimmunoassay (RIA) for bovine TSH (bTSH). Double RIA was performed with purified bTSH and specific bovine antiserum. Laboratory validation included research of minimal detection limit, accuracy, and reproducibility. The physiologic validation included a thyrotropin-releasing hormone (TRH) challenge performed on euthyroid cows and a follow-up of bTSH concentration over a 24-hour period. Furthermore, bTSH concentration was assayed in a large population of healthy dairy and beef cows to define reference interval. The pathologic validation was made by assaying bTSH and thyroid hormones on healthy and goitrous newborn calves. The minimum detection limit (MDL) for bTSH assay was 1.3 mU/ml. The recovery was 101% to 106%. The intra-and interassay coefficients of variation (CVs) ranged from 5% to 11% and 11% to 15%, respectively. The RIA covered the whole range of physiologic bTSH values, as shown by bTSH values induced by TRH-challenge. A pulsatile secretion of bTSH was observed, accompanied by a diurnal variation with lower night values than day values. Reference intervals of bTSH ranged from 1.3 to 13.0 mU/ml for beef and dairy breeds. Finally, bTSH easily discriminated goitrous newborn calves from healthy ones, leading to the definition of a cutoff value of 35 mU/ml. The bTSH assay positively reacted to physiologic and pathologic conditions. The accuracy and precision of the RIA were satisfying.

Effect of a combined iodine and selenium supplementation on I and Se status of cows and their calves

“…A decrease in TSH following supplementation was also observed here, even though TSH values were not significantly different between the HISe and LISe groups and remained within normal range [11,21]. Despite one study [26] suggesting that both T4 and T3 might act directly on the pituitary gland to inhibit TSH secretion, most authors consider that T3, more so than T4, exerts its inhibitory effect on TSH secretion.…”

“…When compared to values found in healthy animals reported in the literature [11,21,22], blood hormone concentrations in the LISe group at the end of the trial did not indicate a deficiency in I which would have resulted in decreased T4 and T3 and increased TSH. Results are expressed as mean ± standard deviation.…”

“…The design of this study limited the effects of physiological factors on hormone levels. Blood was sampled at a fixed time of day in order to avoid the potential variation effect of circadian rhythms on thyroid hormones [18] and TSH [11]. Lactation induces changes in levels of thyroid hormones [19]; thus the cows chosen for this study were non-lactating.…”

“…Plasma samples were analysed for TSH using a radioimmunoassay (RIA) method [11]. Concentrations of thyroid hormones T4, T3 and rT3 were assayed in plasma using diagnostic kits (Clinical Assays TM GammaCoat TM M Total T4 125 I RIA Kit; Total T3 125 I RIA Kit, DiaSorin, Minnesota, USA; Reverse T3, Adaltis, Casalecchio di Reno, Italy).…”

“…An increase in thyrotropin (TSH) levels has been proven to be sensitive enough in mammalian species to be useful in evaluating thyroid function. Recently, reference values for TSH and T4 have been defined in cattle [11].…”