Actually it depends on what level was low. If her TSH
was low it means she is actually hyperthyroid. In this case her T4 would have been high. Your TSH is the hormone put out by your pituitary gland
. T4 is the hormone put out by the thyroid iteself.
If it was her T4 that was low then she is likely hypothyroid, and her TSH would have been high.
One easy way to tell is if she is overwieght, has dry hair, and skin and is irratible it is hypothyroid.
If she is skinnier, hyperactive and her eyes may look a little bug eyed she is hyper thyroid.
Your results are inconclusive until we know what value.
The above answers may be wrong.