This study is about stopping a medicine called **tiratricol** in boys who have a rare condition called **MCT8 deficiency**. MCT8 deficiency is a genetic disorder affecting how thyroid hormones are transported in the body. Tiratricol is a treatment that helps manage thyroid hormone levels. The study is **double-blind**, meaning neither the doctors nor the patients know who is getting tiratricol or a **placebo** (a pill with no medicine) for 30 days. Participants will be checked to see if their thyroid hormone levels change when tiratricol is stopped. The study uses a special test called **LC/MS/MS** to measure these levels. - **Study Duration**: 30 days with a possibility of stopping early if hormone levels go too high. - **Participation Requirements**: Boys 4 years or older with MCT8 deficiency, stable on tiratricol treatment. - **Risks**: Hormone levels may rise if tiratricol is stopped. Eligible participants must have confirmed genetic tests and meet specific health conditions to join the study. This research will help understand the effects of stopping tiratricol in managing MCT8 deficiency.