Low estrogen or thyroid?

My wife has symptoms that could be low estrogen or a thyroid issue. Her symptoms are thinning hair, fatigue,fast heart rate from time to time and tired most times. We just had a baby and she is nursing however, she’s had the symptoms before we got pregnant. Can her doctor check the thyroid and estrogen levels even while she is nursing the baby? I was told that the labs can come back abnormal because she is nursing.
I forgot to mention her thyroid levels always come back normal.