Pregnancy is characterized by significant changes in maternal metabolism that are essential for adequate fetal growth and subsequently lactation. These changes are driven by conceptus demand and maternal supply that is under hormonal control. Here we use a bovine model with purebred and reciprocal cross Bos taurus taurus (Bt, Angus) and Bos taurus indicus (Bi, Brahman) concepti in primiparous dams to demonstrate significant effects of conceptus sex and genetics on maternal circulating thyroid hormone and insulin-like growth factor concentrations. We analysed total and free thyroxine and triiodothyronine, reverse triiodothyronine and insulin-like growth factors 1 and 2 as well as total IGFBPs at midgestation (Day153). In Bt and Bi dams total thyroxine concentrations were significantly higher (P<0.05 and P<0.01) when carrying a male conceptus while triiodothyronine was not affected by conceptus sex (P>0.10). However, free thyroxine was higher in Bt dams carrying females (P<0.05), while Bi dams showed a tendency for the opposite effect (P<0.10). In Bt dams, insulin-like growth factors 1 and 2 were not affected by conceptus sex (P>0.10), but in Bi dams IGF1 tended to be higher (P<0.10) with a male conceptus. Furthermore, we found significant effects of conceptus genetics (BtxBi versus BixBi) on total thyroxine (P<0.001) and a tendency for effects on reverse triiodothyronine (P<0.10) in Bi dams only. Apart from the likely impact on dam and conceptus growth, our data suggest that conceptus sex and genetics may affect hormones that are crucial for mammary gland development and lactation. The current data therefore provide a physiological basis for reported conceptus sex effects on lactation in mammals (Hinde et al. 2014, PLoS One 9(2):e86169) while the impact of dam genetic background may help explain conflicting results (Græsbøll et al. 2015, PLoS One 10(4):e0124051).