This variable shows the date when the beneficiary first met the criteria for the Chronic Conditions Warehouse (CCW) hypothyroidism indicator. The variable will be blank for beneficiaries that have never had the condition.
The earliest possible date for this variable is January 1, 2016, since the algorithm for this condition requires ICD-10 diagnosis codes within the specified lookback period. If the beneficiary became eligible for Medicare after that, the earliest possible date will be some time after his/her coverage start date (i.e., the COVSTART variable in the Master Beneficiary Summary File [MBSF]).
Source: CCW (derived)
From CY 2017 to CY 2021, the MBSF: Chronic Conditions segment also has a previous (CC27) version. The algorithm used to create the First Occurrence of Hypothyroidism variable in the CC27 version includes fewer DX codes and a reduced reference period.