Production on the show was halted in July (09) to allow Tierney to undergo a "medical evaluation" after she had been diagnosed with breast cancer.
But now the actress has ruled out a return to work and her role will be recast.
A spokesman for the show says, "While we are saddened that Maura Tierney won’t be able to continue in her current role in Parenthood, our main concern is for her quick and full recovery. Our thoughts and best wishes are with her."
The actress' representative tells TV show Access Hollywood, "Ms. Tierney and her doctors remain confident that the outcome of her treatments will be positive."