ABSTRACT
Colombia has the greatest number of native bovine breeds from Bos taurus origin in America, which is an important source ofgenetic resources and biodiversity conservation. Those breeds have showed adaptation to the tropic conditions, which arecharacterized by high temperature and humidity, as well as intense solar brightness. Physiological adaptation under heat stressconditions is partially regulated by thyroid hormones. In this work there were studied blood concentrations of thyroid hormones.Twenty-eight cows of eight different Colombian native breeds were used, divided in four different production groups: heifers,early lactation cows, end lactation cows and dry cows. The breeds were: Blanco-Orejinegro (BON), Chino Santandereano (CHI),Hartón del Valle (HVA) and Lucerna (LUC), with dairy production ability; Romosinuano (ROM) and Sanmartinero (SAM) with meatproduction ability; and Casanareño (CAS) and Costeño Con Cuernos (CCC) with undefined production ability. Triiodothyronine(T3) and thyroxine (T4) were measured in serum by radioimmunoassay technique (RIA). Mean values found were 1.89 nmol/Land 49.3 nmol/L, respectively. There were detected significant differences among breeds. Heifers had the greatest concentrationsof thyroid hormones among the groups of animals studied.