Actress Sally Field has thrown her support behind the Human Rights Campaign by opening up about raising a gay son in a heartfelt open letter.

The Lincoln star reveals it was difficult to see her son struggling with his sexuality and wanted to help him, but her two other boys insisted he needed to deal with the difficulties in his own way.

She writes, "Sam is my youngest son, by 18 years, and he's gay. To that, I say: So what? Growing up, Sam wanted desperately to just be like his older brothers... But Sam was different. And his journey to allow himself to be what nature intended him to be was not an easy one.

"When I saw him struggling, I wanted to jump in. But his older brothers held me back. They told me I couldn't travel that road for Sam. It was his to travel, not mine. I had to wait for him to own himself in his own time. I could make it easier only by standing visibly to the side, clearly loving him, always being there and always letting him know."

When Sam turned 20, he revealed he was gay and the 67 year old admits it has been a privilege to be able to support him and the Lesbian, Gay Bisexual and Transgender (Lgbt) community.

And she is calling on others to do the same.

She continues, "Sam knows that if he ever marries, he'll have my full support. After all, I like to believe I raised him with the good sense to choose a great partner.

"But there are people out there... who would rather devote themselves to denying his happiness. Why would anyone want to prevent my son - or anyone's son or daughter - from having basic legal safeguards... It doesn't make any sense - but it won't change until people speak out. I'm proud to stand with Hrc to add my voice. Will you join me?

"Whether you are Lgbt yourself, a parent or grandparent of an Lgbt child, or just a great person with strong convictions about what's fair and right, I hope I've convinced you to stand with Hrc for equality. You'll be glad you did!"