Seasonal changes in weight gain, growth hormone, and thyroid hormones in male red deer (Cervus elaphus atlanticus)

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Abstract
To elucidate possible endocrine mechanisms in the control of the weight cycle of red deer, we recorded seasonal changes in the concentrations of growth hormone (GH), thyroxine (T4), and triiodothyronine (T3) in serum. T3 and T4 were analyzed with specific radioimmunoassays, GH was analyzed with a hetereologous radioimmunoassay. Changes in the levels of these hormones were compared with changes in weight gain. T3 levels were high in summer and low in winter, corresponding to changes in weight gain. Peak values (1.75–4.82 nmol/L) were found in May or June, nadirs occurred in April (0.22–1.78 nmol/L) or October–November (0–1.19 nmol/L). GH levels increased to peak values, 20–53 ng/mL, in April to June, but were low during most of the period of fast weight gain. There were no consistent changes in T4 levels corresponding to the weight cycle. We conclude that the GH peaks following the casting of old antlers may be related to the onset of antler growth. Seasonal changes in T3 levels may be secondary to changes in food intake.