Suburban Bancorp., a giant suburban Maryland bank holding company, reported record year-end and fourth-quarter earnings yesterday.

Net income after securities transactions was $17.7 million ($3.72 a share) compared with $16.7 million ($3.53) in 1980. Assets for the year rose from $1.48 billion to $1.66 billion, and deposits grew from $1.26 billion to $1.30 billion. Loans, however, decreased from $844 million to $842 million.

Fourth-quarter net income after securities transactions was $5 million ($1.05) compared with $4.7 million (99 cents) for the same period last year.

One of the reasons for the company's success was a 23 percent increase in noninterest income and holding noninterest expenses to a 5.9 percent increase over the previous year's level, said Chairman Robert F. Tardio. The company also intensified efforts to place all funds in assets at positive spreads and with similar maturities, Tardio said.

In addition, the company declared a regular quarterly dividend of 42 cents a share, an increase of 7 cents, payable Jan. 29 to shareholders of record Jan. 22.

Central National Bank of Maryland also announced record earnings for 1981. Net income after securities transactions was $615,370 ($2.46 a share) compared with $507,490 ($2.03) for 1980. Assets were $74.9 million compared with $70.1 million. Deposits rose from $63.2 million to $65.3 million, and loans grew from $38.6 million to $44.0 million in 1980.