As many as 13 million Americans may have an undiagnosed thyroid disorder, according to findings from the Colorado Thyroid Disease Prevalence Study. This large screening study, conducted in 1995 by Knoll Pharmaceutical, evaluated thyroid function in more than 25,000 participants.
Among individuals who were not taking thyroid medication, nearly 9% were found to have hypothyroidism, while just over 1% were hyperthyroid. When these findings were extrapolated to the U.S. population, researchers estimated that approximately 13 million Americans could be living with an undiagnosed thyroid condition. The study also reported that even subclinical hypothyroidism—where thyroid hormone levels are only mildly abnormal—was associated with higher cholesterol levels.
Additional evidence suggests that subtle thyroid dysfunction may also be relevant to mental health. A small study published in the American Journal of Psychiatry (1993;150(3):508–510) examined 16 patients with subclinical hypothyroidism and 15 individuals with normal thyroid function. The researchers found that lifetime rates of depression were higher in the group with subclinical hypothyroidism compared with those with normal thyroid status.
Together, these findings suggest that mild or unrecognized thyroid dysfunction may influence both metabolic markers (such as cholesterol) and mood-related outcomes. While these studies do not establish causation, they highlight the potential importance of thyroid function in broader health assessments.
For a broader overview of thyroid symptoms, testing, and thyroid disorders, see Thyroid Health: Symptoms, Testing, and Common Thyroid Disorders.
