Annual variations in serum thyroid-stimulating hormone and thyroid hormones and in their responses to thyrotrophin-releasing hormone in the reindeer

Abstract

The reindeer in its natural habitat is subject to great annual variations in ambient temperature, illumination and nutrition. To ascertain the effect of these environmental factors on thyroid function, serum thyroid-stimulating hormone (TSH), thyroxine (T4), tri-iodothyronine (T3) and reverse T3 (rT3) concentrations were measured four times a year (2 June, 8 October, 21 November, and 24 February) in 14 animals housed outdoors at latitude 69 degrees 10'N. They all showed statistically significant (P < 0.05) seasonal changes. Serum TSH and T4 were highest in February (623 +/- 30 ng/ml and 287 +/- 19 nmol/l respectively). TSH was lowest in October (318 +/- 47 ng/ml) and T4 in November (199 +/- 19 nmol/l). The T3 concentration was highest in November (3.0 +/- 0.3 nmol/l) and lowest in June (1.8 +/- 0.2 nmol/l). In contrast, rT3 was highest in June (3.6 +/- 1.2 nmol/l) and lowest in November (1.9 +/- 0.6 nmol/l). Thus, there was an inverse relationship between T3 and rT3 (linear regression r = -0.406, P < 0.01). TSH, T4, T3 and rT3 responses to exogenous thyrotrophin-releasing hormone (synthetic TRH; 500 micrograms i.m.) were determined in ten animals. The magnitude of their response to TRH was significantly (P < 0.05) dependent on the time of year. When compared with the control level all the parameters rose significantly (P < 0.05). The greatest rise in serum TSH occurred in October (219 +/- 151%) and the smallest in February (66 +/- 53%).(ABSTRACT TRUNCATED AT 250 WORDS)

Growth responses of broiler chickens to different periods of artificial light1

“…Overall, these hormones increase the basal metabolic rate to make more glucose available to cells to stimulate protein synthesis, increase lipid metabolism, and simulate cardiac and neural functions (Todini et al, 2007). Moreover, plasma thyroid hormone concentrations are correlated with feed intake in ruminant species (Ryg and Langvatn, 1982;Chunchin and Brown, 1984;Timisjärvi et al, 1994). In another study, correlations Table 3.…”

Seasonal changes in reindeer physiology

“…In reindeer controversial results have been reported on the seasonal changes in T3 and T4 concentrations with a nadir occurring either in winter or summer while others have found no seasonal variation (Ryg & Jacobsen, 1982;Nilssen et al, 1985;Timisjärvi et al, 1994;Bubenik et al, 1998). Thus it has been concluded that changes in the concentrations of thy- roid hormones do not indicate variations in basal metabolic rate, but they rather fluctuate according to feeding (Ryg & Jacobsen, 1982;Ryg, 1984;Nilssen et al, 1985).…”

Temporal and spatial variation in plasma thyroxine (T4) concentrations in juvenile alligators collected from lake Okeechobee and the northern Everglades, Florida, USA

“…Non‐endocrine‐induced variation in thyroid hormone concentrations has been documented in fish, amphibians, reptiles, and mammals, thus stressing the complex relationship between thyroid activity and environmental factors. Thyroid hormone concentrations have been demonstrated to vary seasonally [2–11], with both photoperiod and temperature influencing plasma concentrations. Temperature influences thyroid activity in reptiles, although activity generally is lower than that observed in mammals [12].…”