Interactions between the thyroid, stress and the immune system of the domestic fowl (Gallus domesticus)
The involvement of the thyroid hormones and corticosterone in the response to stressors, the influence of these hormones on the immune system and their possible role in the stress-induced changes in immune function, were investigated. Food and/or water deprivation, food restriction, exposure to heat or cold, all influenced serum thyroid hormone concentrations. Changes in food intake probably influenced serum concentrations of T3 and kT4 by reducing extrathyroidal 5 and 5' monodeiodination of T4. The effects on thyroid hormone concentrations, of heat, and possibly also cold and deprivation of water, were in part mediated by food intake. Deprivation of food or water, food restriction and cold all cause increases in circulating corticosterone. Though corticosterone influenced thyroid function in unstressed birds, it did not mediate the effects of underfeeding on serum T3 and T4. Circulating corticosterone was not increased during heat exposure, though changes in other physiological variables indicated a severe stress. It is concluded that circulating corticosterone cannot be relied upon as the sole index of stress. Thyroid hormones did not markedly affect humoral immunity, whereas corticosterone suppressed antibody production, probably by reducing T-helper cell function. Both thyroid hormones and corticosterone influenced cell-mediated immunity as evinced by changes in lymphoblastogenic responses to PHA and Con A. Food restriction, heat exposure and social stress suppressed PHA and/or Con A responses. However, these effects were not accounted for by the observed changes in thyroid hormones or corticosterone. It is concluded that although the thyroid hormones and corticosterone can influence cell-mediated immunity in unstressed birds, they are not directly responsible for the immuno-suppression associated with food restriction, heat or social stress.