<p>Seasonal changes in horn growth and testicular activity in wild bovid are mainly modulated by the photoperiod. Thyroid hormones are required for the transition from the rutting season to the end of the breeding season in domestic ruminants, but their role on modulation the seasonal horn growth in wild bovid is not clear. The purpose of this study was to test the hypothesis that thyroid hormones are required for photoperiodically driven changes in horn growth and testicular activity of the mouflon as a model of wild ruminants. Plasma concentrations of thyroxine were reduced via treatment with propylthiouracil (PTU) under different photoperiodic conditions (natural photoperiod and artificial long days—LDP) from 1 November to 31 December. Plasma testosterone, plasma thyroxine, and horn growth were measured throughout the experimental period (November-March). The overall horn growth during the experiment was higher (<i>p</i> &lt; 0.05) in LDP group than controls, PTU and PTU-LDP groups. The proportion of motile sperm, viable sperm, and sperm with an intact acrosome were reduced (<i>p</i> &lt; 0.05) in PTU group in comparison with control. PTU-LDP showed the greatest progressive motility, straight-line velocity, beat-cross frequency, and linearity of sperm (<i>p</i> &lt; 0.05). Morphological abnormalities were lower (<i>p</i> &lt; 0.05) in LDP group than control. In conclusion, the stimulatory effect of the long-day photoperiod on horn growth requires adequate levels of thyroxine, which is also required for the transition from the rutting season to the end of the breeding season in mouflon.</p>