Thyroid hormones in donkey blood and milk: correlations with milk yield and environmental temperatures

Thyroid hormones (TH) are the primary endocrine stimulators of non-shivering thermogenesis and are known to stimulate lactation. Triiodothyronine (T3) is the bioactive form, mainly derived by deiodination of thyroxine (T4), and the free quote (unbound to plasma proteins) is immediately bioavailable. This study aimed to evaluate potential relationships among TH in the blood, triiodothyronine in the milk (T3M), milk yield and environmental temperature in March to July for 8 lactating donkeys. Milk yield and blood TH concentrations changed significantly over time, whereas T3M was rather stable among individuals and not affected by time of sampling. Free T3 was not correlated with free T4 or with total TH in the blood, but was weakly correlated with T3M. No relationship was found between blood TH and milk yield, which was negatively correlated with T3M, thus the absolute quantity of bioactive hormone in milk secretion is maintained. Milk yield was positively correlated with the free/total T3 and free T3/free T4 ratios and, thus, with the relative quote of the circulating bioactive hormone. Circulating T3/T4 ratios were negatively correlated with environmental temperature. It is concluded that environmental temperature, in the range of the present study (-2 to 35 °C), does not significantly entrain thyroid gland activity, which is affected more by other factors, such as inter-individual variations and physiological status (i.e., stage of lactation). However, increases in environmental temperature most likely induced decreases in deiodinase activity at the peripheral tissue level, as indicated by the decrease in the T3/T4 ratios in the blood.