Thyroid hormones (TH) and 5'-monodeiodinase (5'-MD) activity in goat's milk from the early, mid- and late lactation period

The physiological significance of thyroid hormones (TH) present in colostrum and milk is still under consideration. The present study was aimed at determining milk thyroxine (T-4) and triiodothyronine (T-3) levels in three lactation phases (early, mid- and late) of the goat, and to measure activity of the milk 5'-deiodinase (5'-MD) enzyme responsible for the intramammary conversion of prohormone T-4 to its metabolically highly active form T-3. Thirty-two milk goats (Polish White breed) fed a standard diet were used for milk sampling. The highest TH levels in mammary secretion were recorded during the first 2-3 days post partum. Then the hormone levels decreased, and by about Day 7 fluctuated around the overall mean for the early-lactation phase (Days 1 to 24 of lactation), recording 0.134 +/- 0.059 mug T-4 and 150.8 +/- 2.80 ng T-3 in 100 ml of the milk. Such T-4 concentrations appeared to be comparable to those in the rabbit and human, whereas the concentration of T-3 was higher than in the cow, pig and mare's milk. Milk 5'-MD activity was higher (P < 0.01) during early and late lactation, compared to the mid-lactation phase. It coincided with low T-4 and high T-3 milk levels during early lactation, and with high milk T-4 and low T-3 concentrations during late lactation. The quantity of T-4 and T-3 available to newborn kids in milk suggests that TH ingested with the colostrum may have a physiological role during the early postnatal life of suckling goats.