Is it ture that with the mirena iud that your pregnancy test will come up negative even if you are pregnant because of the hormones in the iud?

No, it's not true.

I actually have a colleague who is pregnant with a mirena in place. She got a positive test at 4 weeks gestation.

Pregnancy results in the production of the hormone called Hcg. The mirena has a low level of the hormone called progestin. Progestin will not conteract the pregnancy hormone in a pregnancy test.