This is a clinical study in two phases. The first phase compares the effect of an ointment containing parathyroid hormone (PTH) with the effect of a placebo ointment (inactive ointment without PTH) on psoriasis lesions. Neither the study participants nor the researchers will know who is receiving PTH ointment and who is receiving placebo until the end of this first study phase. The second phase is a study of the PTH ointment on large areas of psoriasis to find out how long the effects last.