TD Ameritrade, the US online broker, signaled its intention to expand its presence in derivatives trading on Thursday by agreeing to take over thinkorswim, an online options broker, in a cash-and-shares deal worth $606m.

The proposed deal – which gives thinkorswim shareholders $3.34 in cash and 0.398 of an Ameritrade share plus for each thinkorswim share – values thinkorswim at $8.71 per share, 54 per cent more than Wednesday’s closing price of $5.65.