Background: Propylthiouracil (PTU, 6-n-propyl 2-thiouracil) is an antithyroid thioureylene, which, in addition to its ability to decrease thyroid hormone synthesis, also has immune modulatory and free radical scavenging abilities. We have previously shown that oral PTU and another antithyroid thioureylene are effective in the treatment of plaque psoriasis.

Objective: The current study was performed to determine the efficacy of topical PTU in psoriasis.

Methods: Topical PTU and placebo were administered, in a double-blind fashion, three times daily for 4 to 8 weeks to nine volunteers with long-standing plaque psoriasis. The patients had biopsy specimens of their lesions taken at the start and end of the study, Clinical response was monitored with a scoring system based on scale, erythema, and thickness of the plaques. Complete blood cell count and thyroid function studies were obtained in each patient at the beginning and at 2-week intervals thereafter until completion of the study.

Results: Topically applied PTU produced significant clearing of the lesions (clinical scores 8.0 ± 0.6 vs 3.7 ± 0.3, p < 0.0001 at 4 weeks, and 4.0 ± 0.6, p < 0.02 at 8 weeks); two patients demonstrated nearly complete clearing. Placebo-treated and untreated “control” areas showed no significant change during the study. None of the subjects had hypothyroidism or cytopenia.

Conclusion: Topical applied PTU is effective in the treatment of patients with stable plaque psoriasis and has low toxicity.