Thyroid u/s showed nodule in isthmus: 9x3x7mm, slightly hypoechoic, markedly vascular. at biopsy, neither dr could locate it. should i be worried?

No: Bad disease is not hard to find. At the current measurement it is still very small. I would do another ultrasound in 1 year. If it's still small or can't be found, do nothing then. No worries.

Try not to.: Get a followup ultrasound in 6 months. Thyroid nodules are very common but these nodules are rarely cancerous. If it gets bigger they may need to try again for needle cytology.