I have hypothryoidism, and my temps are somewhat low, not too bad, but I am treated. I'm sure if I wasn't taking medicine to treat it they would be worse. If that's really the only symptom you have though, it may not be thyroid. It could just be some other hormonal imbalance. Being hypothyroid doesn't always make it hard to get pregnant, especially if you are subclinical. The only way to know for sure is to get the bloodwork done, but it is pretty expensive w/o insurance. You could just get hormonal bloodwork done first, like estrogen and progesterone levels. Those may not be as expensive as a thyroid panel is.
Ultimately you may just want to wait and see if any other symptoms develop or not. If you have always had low BBT, but had no trouble getting pregnant before, that may just be the way your body is. HTH!
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"Let us not become weary in doing good, for at the proper time we will reap a harvest if we do not give up." - Galatians 6:9

TTC on our own for over 2 years
Dx with Severe MFI- Blood work shows no Microdeletion and no CF mutations
Ultrasound scheduled 3/22- DONE! Everything looks normal, other than some cysts on my right ovary.
HSG scheduled 4/14- DONE! Tubes are clear!!

On BCP until our IVF w/ICSI scheduled for August!!!