Valproate is used as an anti-epileptic and as a mood stabiliser. There are some risks of PCOS for women, but those risks have never been explained properly to me.

The connection that immediately leaps to mind is the role of dopamine in inhibiting prolactin—if you have a slight hormone production imbalance on the thyroid/pituitary axis, decreasing dopamine production (and thus increasing prolactin) could exacerbate it.

Valproate causes massive weight gain. It likely exacerbates any conditions (incl. PCOS) associated with obesity.