Subclinical hypothyroidism (SHO) and subclinical hyperthyroidism (SHP) are defined biochemically as a normal serum free thyroxine (FT₄) concentration in the presence of an elevated and diminished serum thyroid-stimulating hormone (TSH) concentration, respectively. The objective of this study was to investigate the changing prevalence of SHO and SHP from April 2004 to March 2014, which analyzed the cases of 386,846 medical checkup patients during the ten year period. SHO and SHP were found in 4.02% and 1.23% of the subjects, respectively. The age-specific prevalence of SHO demonstrated a tendency to increase with age, while that of SHP showed a gradual decrease with age. Although the number of examinees mostly remained unchanged from 2006, the prevalence of SHO showed an increasing trend, and the prevalence of SHP showed a decreasing trend on a yearly comparison. The age group with the most prevalent number of SHO cases was changing by year. In contrast, the number of SHP cases was most prevalent in the 40s age group except for in 2006, and SHP in the 30s age group had decreased remarkably each year. This study concludes that within this 10 year period, there was an increasing trend of SHO prevalence while SHP demonstrated a decreasing trend.