FDA Approves First Treatment for Rare Thyroid Hormone Disorder

The FDA approved tiratricol (Emcitate) to treat peripheral thyrotoxicosis in patients with monocarboxylate transporter 8 (MCT8) deficiency, the agency announced on Monday, marking the first approval for this rare genetic disorder.

The thyroid hormone receptor agonist is indicated for adults and children with MCT8 deficiency, also known as Allan-Herndon-Dudley syndrome.

“The challenge in treating MCT8 deficiency has always been that the protein needed to deliver thyroid hormone into cells is the one that isn’t working,” said Hylton Joffe, MD, MMSc, of the FDA’s Center for Drug Evaluation and Research, in the press release. “This drug sidesteps that problem, as its active ingredient, tiratricol, can enter cells on its own without relying on the broken transporter, leading to a decrease in the elevated blood thyroid hormone levels.”

Approval was supported by two studies: the randomized, placebo-controlled ReTRIACt trial, which enrolled 20 participants ages 5 to 31 years, and an open-label trial of 46 participants ages 10 months to 66.8 years.

Across both studies, tiratricol lowered excess blood thyroid hormone levels and improved cardiovascular and metabolic symptoms impacted by thyroid levels, such as systolic blood pressure and heart rate.

MCT8 deficiency is a rare, life-limiting, X-linked disorder caused by mutations in the SLC16A2 gene that primarily affects males. Because these mutations disrupt the MCT8 cell-surface transporter, thyroid hormone signaling becomes severely imbalanced. The central nervous system becomes deprived of thyroid hormone, while peripheral tissues are overexposed to active T3.

This dual imbalance causes debilitating neurodevelopmental impairment alongside persistent peripheral thyrotoxicosis, often leaving patients unable to sit or walk independently and causing chronic stress on the heart and metabolism.

“Through my experience caring for patients with MCT8 deficiency, I have seen firsthand the profound impact this complex and life-limiting disorder can have on patients and their families,” said Andrew Bauer, MD, principal investigator of ReTRIACt, in a statement from drugmaker Egetis Therapeutics.

The approval “provides physicians in the U.S. with the first approved treatment option for patients and represents an important advance for the MCT8 deficiency community,” he added.

Tiratricol is administered once daily as a liquid suspension, given either orally or through a feeding tube for patients with swallowing difficulties. The most common adverse effects include diarrhea, vomiting, rash, and hyperhidrosis.

The drug’s label carries a boxed warning stating that it is not intended for weight loss or obesity. Prescribing information also cautions against use in patients with primary hyperthyroidism or alongside other thyroid medications, and warns of risks including thyrotoxicosis and interference with laboratory tests measuring T3 levels.

Egetis Therapeutics said that tiratricol will be available in the U.S. within 8 to 10 weeks.