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5'-Deiodinase Activity and Circulating Thyronines in Lactating Cows

C. Pezzi^*, P. A. Accorsi^*, D. Vigo, N. Govoni^* and R. Gaiani^*

^* Dipartimento di Morfofisiologia Veterinaria e Produzioni Animali, Facoltà di Medicina Veterinaria, Università di Bologna, Via Tolara di Sopra 50, 40064 Ozzano Emilia (BO), Italia
Dipartimento di Scienze e Tecnologie Veterinarie per la Sicurezza Alimentare, Facoltà di Medicina Veterinaria, Università di Milano, Via Celoria 10, Italia

Corresponding author:
C. Pezzi; e-mail:
cpezzi{at}vet.unibo.it.

To investigate the correlation between lactation and thyroid hormone metabolism, the authors studied concentrations of total and free thyroxine (T4 and fT4), triiodothyronine (T3 and fT3), and reverse triiodothyronine (rT3) in plasma and milk, as well as liver and mammary gland 5'-deiodinase (5'D) activity in dry, early, middle, and late lactating dairy cows. Cows in early lactation show lower plasma levels of T4 and rT3 than dry, middle, and late lactating animals, whereas T3 shows the lowest plasma levels in the dry period; free T4 and T3 show a similar pattern. In early lactation there is a clear decrease in liver 5'D associated with a notable increase in mammary 5'D. Concentrations of T4 and T3 in milk drop significantly in the first few days after delivery, whereas rT3 increases up to the fourth month. The findings suggest a relationship between the hypothyroid status of lactating cows and the rearrangement of organ-specific 5'-deiodinase activity related to the maintenance of the udder’s function.

Key Words: dairy cows • mammary gland • liver • thyroid hormones

Abbreviation key: T3 = 3,5,3'-triiodothyronine, fT3 = free triiodothyronine, T4 = thyroxine, fT4 = free thyroxine, rT3 = reverse triiodothyronine, 5'D = 5'-deiodinase, D1 = type I 5'-deiodinase, D2 = type II 5'-deiodinase, PTU = propylthiouracil, DTT = dithiothreitol

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