What causes inner thigh itching?

Try antifungal. Inner thigh itching could be caused by many factors. A trial of otc anti fungal medicine, such as terbinafine (lamisil) applied twice daily is a reasonable action. Its always wise to see your doctor or a dermatologist for evaluation if concerned or if it doesn't improve.
Yeast. Usually it is yeast esp if your gut ecology is altered simple remedy use one of the following daily - garlic supplement, propolis, oregano supplements or caprilic acid(coconut oil extract) to get rid of gut yeast - locally use any otc cream with antifungal meds.