FDA Approves Tiratricol, First Treatment for MCT8 Deficiency

The FDA has approved tiratricol (Emcitate; Egetis Therapeutics) for the treatment of peripheral thyrotoxicosis in adults and pediatric patients with monocarboxylate transporter 8 (MCT8) deficiency, also known as Allan-Herndon-Dudley syndrome, according to a news release from the FDA and Egetis Therapeutics. It is the first FDA-approved therapy for this rare genetic disorder.^1,2

The approval gives clinicians a way to lower the excess circulating thyroid hormone that strains the heart and metabolism in these patients. It also brings monitoring, dispensing, and drug interaction considerations that fall to pharmacists.^1,2

Bypassing a Broken Transporter

MCT8 deficiency is an X-linked disorder, caused by pathogenic variants in the SLC16A2 gene, that primarily affects males. Without a functioning MCT8 transporter, thyroid hormone cannot adequately reach the brain, while triiodothyronine (T3) builds up in the bloodstream. Patients may be unable to walk or sit independently and may have absent or severely limited speech, intellectual disability, and feeding difficulties. Egetis reports a median life expectancy of approximately 35 years.^1,2

Tiratricol is a thyroid hormone receptor agonist. Unlike T3 and thyroxine, it can enter cells without relying on MCT8, which lets it modulate hypothalamic-pituitary-thyroid axis signaling and reduce circulating T3.^1,3

ReTRIACt and Long-Term Data

Approval was supported by 2 studies. In the phase 3 ReTRIACt trial (NCT05579327), 20 male patients aged 5 to 31 years entered a run-in or titration period. The 15 who reached a stable dose were randomly assigned to continue tiratricol or switch to placebo for 30 days. The ratio of the rate of change in total T3 favored continued treatment (1.5; 95% CI, 1.0-2.2; P = .034). Mean total T3 rose 64.6 ng/dL with placebo vs –0.2 ng/dL with tiratricol, and every patient given placebo had a larger T3 increase than any patient who stayed on treatment. On the second primary end point, 4 of 8 patients receiving placebo met the rescue criterion of total T3 above the upper limit of normal, compared with 1 of 7 receiving tiratricol. That 1 patient discontinued early and was counted as meeting the criterion. The 95% CI for the difference between groups crossed zero.^3,4

In a 12-month, open-label, single-arm study of 46 male patients aged 10 months to 66.8 years, mean total T3 fell from 323.4 ng/dL at baseline to 118.3 ng/dL. Heart rate decreased by a mean of 8.9 beats per minute and systolic blood pressure by a mean of 4.1 mm Hg.³

Monitoring and Interactions

Tiratricol cross-reacts with T3 immunoassays and can falsely elevate results, so the label directs clinicians to measure total T3 by liquid chromatography tandem mass spectrometry (LC-MS/MS). No FDA-authorized LC-MS/MS test for total T3 is currently available, and existing tests may vary in accuracy.³

Tiratricol cross-reacts with both total and free T3 immunoassays, according to Andrew J. Bauer, MD, director of the Thyroid Center in the Division of Endocrinology and Diabetes and professor of pediatrics at the Perelman School of Medicine at the University of Pennsylvania, who was a principal investigator in ReTRIACt.^2,5

"Thus, total or free-T3 by liquid chromatography-tandem mass spectrometry must be utilized to adjust the dosing of [tiratricol]," Bauer told Pharmacy Times. "The dose is targeted to lower the T3 or free-T3 in the lower half of the age-specific reference range."

Starting doses are 175 µg/d for patients weighing 10 kg or less and 350 µg/d for those weighing more. Doses are titrated about every 2 weeks toward a total T3 level below the midpoint of the age-specific normal range. Signs of thyrotoxicosis, including increased heart rate, elevated blood pressure, diarrhea, and hyperhidrosis, may emerge during initiation and titration. The drug should be tapered rather than stopped abruptly. It carries a boxed warning against use for obesity or weight loss.³

Concomitant thyroid medications such as levothyroxine should be avoided, as should bile acid sequestrants and psychostimulants. Other interaction guidance:

• Iron and calcium supplements should be given at a consistent time relative to each dose.
• T3 should be monitored when a proton pump inhibitor is started or stopped.
• Patients taking oral anticoagulants need close coagulation monitoring.

The most common adverse reactions were diarrhea, vomiting, rash, and hyperhidrosis.³

Preparation and Access

The 350-µg tablets must be dispersed in room-temperature drinking water in a 10- to 12-mL syringe. Any suspension not used within 30 minutes should be discarded, and tablets should not be swallowed whole, chewed, or crushed. Tablets, including split halves, require refrigeration and protection from light. Egetis expects commercial availability in 8 to 10 weeks, with distribution through the specialty pharmacy PANTHERx Rare and its RareLink patient support program.^2,3

Bauer noted that instructions for preparing and administering the suspension at home will be included with the prescription when it is picked up from the pharmacy. "The suspension will be cloudy and will contain small particles," he said. "The suspension can be administered by mouth for patients that are allowed oral intake or via a gastric tube."

REFERENCES

1. FDA approves first treatment for MCT8 deficiency. News release. FDA. September 28, 2026. Accessed September 29, 2026. https://www.fda.gov/news-events/press-announcements/fda-approves-first-treatment-mct8-deficiency

2. Egetis Therapeutics announces U.S. FDA approval of Emcitate (tiratricol) for patients with MCT8 deficiency. News release. Egetis Therapeutics. September 28, 2026. Accessed September 29, 2026. https://www.egetis.com/news/egetis-therapeutics-announces-u-s-fda-approval-of-emcitate-tiratricol-for-patients-with-mct8-deficiency/

3. Emcitate (tiratricol) tablets. Prescribing information. Egetis Therapeutics US; revised September 2026. Accessed September 29, 2026. https://www.egetis.com/wp-content/uploads/emcitate-full-prescribing-information.pdf

4. Egetis announces positive results from the ReTRIACt study of Emcitate (tiratricol) in MCT8 deficiency. News release. Egetis Therapeutics. November 14, 2025. Accessed September 29, 2026. https://www.egetis.com/mfn_news/egetis-announces-positive-results-from-the-retriact-study-of-emcitate-tiratricol-in-mct8-deficiency/

5. Chan SL, Refetoff S, Babic N, Jin M, Garg U, Yeo KJ. Triiodothyroacetic acid cross-reacts with measurement of triiodothyronine (T3) on various immunoassay platforms. Am J Clin Pathol. 2022;157(2):156-158. doi:10.1093/ajcp/aqab124