The three most common labs measured to assess thyroid function are TSH (thyroid stimulating hormone), Free T4 (thyroxine), and Free T3 (triiodothyronine).
Thyroid conditions such as hypothyroidism (underactive thyroid) and hyperthyroidism (overactive thyroid) are more common in people with Down syndrome. Current guidelines for adults with Down syndrome recommend checking for thyroid conditions with a blood test every 1-2 years.
Source: Brian Chicoine, MD