Hypothyroidism was believed to be a rare and potentially fatal clinical entity when it was first described more than a century ago.1 With the subsequent development of techniques to measure thyroid hormone levels in serum, hypothyroidism was shown to be a common disorder, with a broad spectrum of severity. But it was not until the early 1970s, when thyrotropin (thyroid-stimulating hormone [TSH]) measurements became routine, that we learned just how common the disorder and how broad the spectrum.2,3
We now know that the mildest form of hypothyroidism, manifested solely by an elevated serum TSH concentration, is present in approximately 5% of the population, with a prevalence in women older than age 60 years that approaches 15%.4,5 Of 28.5 million people older than age 65 years in America, 17 million of whom are women, an estimated 3.5 million have mild thyroid impairment, appropriately termed subclinical hypothyroidism. Autoimmune thyroid