Shares move 2.4% higher to 149p on phase III clinical trials showing that Infacort increases the cortisol hormone in paediatric Adrenal Insufficiency sufferers.
The condition causes weight loss, muscle weakness, fatigue and low blood pressure due to the adrenal glands not producing enough cortisol.
If approved for sale by the regulator this would be the first such treatment for children under six years old suffering from the condition.
Stockbroker Numis expects the treatment to be launched in Europe in 2018 assuming European approval is granted in the next year.
Diurnal has another treatment called Chronocort under development, which targets congenital adrenal hyperplasia. The condition causes infertility and development defects such as stinted growth and genital deformities.
Chronocort is expected to be launched in Europe in 2019.
Infacort and Chronocort have received orphan drug status from US regulator the Food & Drug Administration (FDA). This provides longer periods of exclusivity for treatments developed to tackle rare conditions, while tax breaks could reduce costs.
Management could also benefit from higher prices from having the first drugs on the market for these particular conditions.
Numis expects these products to generate £21 million of sales by 2020, rising to £237 million by 2024.