Improved Diagnosis of Mild Hypothyroidism Using Time-of-Day Normal Ranges for Thyrotropin

Either 8 am TSH or 4 pm TSH (compared with time-of-day normal range) can identify TSH elevation. Low am/pm TSH ratio (FT4 in lowest one-third of normal) confirms central hypothyroidism. Thus, time-of-day TSH ranges should be used for accurate diagnosis and more appropriate cost-effective treatment of mild hypothyroidism.