Thrift institutions reduced their holdings of mortgage securities during the second quarter, making room in their investment portfolios for more desirable unsecuritized ARMs. The Office of Thrift Supervision reported that institutions it regulates held $146.74 billion of various mortgage securities at the end of June, down 3.9 percent from the previous period. Thrift MBS holdings had been generally trending lower until the first quarter of this year, when they spiked up to $152.69 billion, the highest…