What is considered a fast rate of growth for a thyroid nodule that is solid?

Thyroid. Thyroid nodule that is solid needs further evaluation irrespective of size.
>4-5 mm in 6 mo. For solid nodules >1cm or nodules which are progressively enlarging (>4-5 mm in 6 mo) i usually recommend needle biopsy. Those characteristics increase the risk of a thyroid nodule of being a thyroid cancer. There are other things to look out for as well on your ultrasound (the best way to look at the thyroid). You should discuss the options for management with an ent/thyroid surgeon.
Thyroid module. Doubling size in 6 months would be a fast rate of progression.