Inefficacy of topical thyroid hormone analogue TriAc in plaque psoriasis: results of a double-blind placebo-controlled trial

Abstract

Background: Thyroid hormone receptors are expressed in human skin and are believed to be involved in the regulation of epidermal proliferation and differentiation, i.e. processes which are disturbed in psoriatic skin lesions. Ligands of the thyroid hormone receptors have so far not been tested as antipsoriatic agents. TriAc (3,3',5-triiodo-thyroacetic acid) is a well-known thyroid hormone analogue with much reduced cardiac thyrotoxic activity compared with the classical thyroid hormones.

Objectives: To determine the effectiveness and side-effects of topical TriAc in patients with chronic plaque psoriasis.

Methods: Twelve patients with mild to moderate psoriasis were treated with TriAc (0.1% in hydrophilic ointment) and placebo applied twice daily to either of two (or several) bilaterally symmetrical plaques for 8 weeks. The patients and investigator were blinded as to the content of the tubes. Every 2 weeks the treated plaques were evaluated by the patient (using a balanced visual analogue scale for a right-left comparison) and by the investigator (using a psoriasis severity index and a global assessment of each plaque).

Results: After 8 weeks of treatment, more than 33% improvement of the psoriasis index occurred in 10 of 12 TriAc-treated and nine of 12 placebo-treated plaques. There were no statistically significant differences between the treatments in terms of reduction of the scores for erythema, scaling, induration or pruritus during the study. Half of the patients considered TriAc superior to placebo, whereas three of 12 were of the opposite opinion (P > 0.05). The global assessment showed marked improvement or remission in six TriAc-treated and five placebo-treated cases (P > 0.05 for difference). No adverse effects were noted.

Conclusions: TriAc in the dosage and formulation studied was safe but no more effective than placebo in treating plaque psoriasis. However, newer thyroid hormone analogues (agonists or antagonists) might be more active and should be further explored in this context.