Is a heterogeneous thyroid nodule cancerous?

Possibly: Each thyroid nodule has an approx. 5% chance of malignancy, thus a heterogenous could be cancerous. Looking at it the other way, a heterogenous nodule has a 95% chance of not being cancerous.

Not necessarily...: There are many factors that help to gauge one's suspicion of a thyroid nodule being cancerous or not: is the lesion growing or stable? Is it heterogenous or homogenous? Is it cystic or solid? Does it have calcifications? Does it have regular borders? Etc.... In the end, a biopsy (perhaps with genetic analysis) would be the best way to determine the malignant potential of a thyroid nodule.

Not necessarily: Not necessarily. Lots of nodules are heterogeneous and lots of those are benign. In fact most of them are benign.

Heterogenous nodule: Although heterogeneity is a feature of malignant thyroid nodules, it's not an absolute. There are several features on ultrasound which suggest malignancy but all should lead your doctor to performing or ordering an ultrasound guided biopsy.