The composition of culture media is a key element in the process of in vitro embryo production. With the development of defined culture media, many components that are present in trace amounts in follicular fluid and serum have been excluded from the in vitro embryo production system. Among these are hormones, which have important regulatory roles in growth, metabolism and differentiation and are known to be present in follicular fluid, serum and the female reproductive tract. We have investigated the effects of supplementation of in vitro maturation and/or culture medium with testosterone (T), androstenedione (A4) and thyroid hormones (TH) on bovine in vitro embryo production and the mechanisms of action of TH in developing embryos. Our results show that testosterone, but not androstenedione or thyroid hormone supplementation increased cleavage rates. None of the treatments significantly altered the sex ratio. Addition of thyroid hormones, T3 and T4, to the in vitro culture media resulted in a significant increase in the rate of development to the blastocyst stage. In addition, blastocysts from the T3/T4 treated groups had higher cell number and lower rates of apoptosis. We have confirmed the expression of mRNAs for both Thyroid hormone receptor α and β (TR α and β) in cumulus‐oocyte‐complexes, oocytes, and in both treated and control blastocyst. Quantitatively, the expression of TR mRNA was higher in the treated embryos but the difference was not statistically significant. TR proteins were detectable in blastocysts of both groups with a difference in the distribution pattern. TH treated embryos had peri‐nuclear concentration of TR while in control embryos it was homogenously distributed in the cytoplasm. Preliminary studies of inhibition of TR α and β by siRNA knockdown by micro injection at the zygote stage show a drastic reduction in development suggesting that TH play an essential role in embryo development. They appear to mediate this effect by their receptors, TR α and β. Overall the results show that the presence of hormones in maturation and the culture medium can alter the outcome of in vitro embryo production and highlight the significance of biological components missing from in defined embryo culture media.