As many as 13 million Americans may have an undiagnosed thyroid problem, according to the Colorado Thyroid Disease Prevalence Study. The study was conducted by Knoll Pharmaceutical (makers of Synthroid) and involved more than 25,000 participants in 1995. Researchers found that nearly 9% of participants who were not taking thyroid medication were hypothyroid, and a little over 1% were hyperthyroid. When these figures were extrapolated to the entire U.S. population, the estimated number of people with undiagnosed thyroid disease approached 13 million. The study also found that even “subclinical” hypothyroidism may be associated with elevated cholesterol levels.