Agreed, low basal temperature usually is suggestive of a thyroid dysfunction. As Frog also mentioned, one problem, common problem, is that batteries of thyroid tests can be done and come up negative. Thyroid is very tricky to test for, and there is not just one test. T3, T4, T7 uptake, and more -- you can be tested repeatedly, without accurate results.
One of the tried and true best indicators thus far has been basal temperature, despite any blood test results. Every morning take your temperature while you're still in bed, before you get up, before you even get OUT of bed, before anything to eat or drink, and write it down. Then take your temperature 4 times per day, every day, and do this for at least 7-10 days. Find the "average mean" of your temperature as well, and if it is chronically low, it *may* be a good indicator that you have some thyroid issues to contend with.