Functioning of the thyroid gland in sheep infected with Trichostrongylus colubriformis

Radioactive tracers were used to to study the kinetic behaviour of inorganic iodine (I) and thyroxine (T4) in sheep (1) uninfected and fed ad libitum, (2) infected with Trichostronglylus colubriformis and fed ad libitum and (3) uninfected but pair-fed to the infected animals. Plasma concentration and pool size of inorganic I were each significantly lower in groups 2 and 3 than in 1. Irreversible loss of inorganic I in both groups 2 and 3 was significantly greater than in 1. The turnover rate of T4 increased after infection, whereas in pair-fed sheep the T4 turnover rate was not significantly different from that in uninfected controls. The pool size and rate of irreversible loss of T4 in infected sheep were significantly reduced. The increased T4 turnover rate and reduced availability of I probably caused the depletion of colloid seen in the thyroid gland of infected sheep. The depletion of colloid may account for the reduced response to thyroid stimulating hormone in these sheep.