Reduction in breast size after childbirth may be due to deficiency of pituitary hormones. It is usually associated with a lack of milk discharge, weakness, weight loss and irregular or no menses.
If it is only a decrease in breast size and nothing else, then wait and watch.
If other symptoms are present, then assessment of pituitary hormones is required. i.e. FT3, FT4, TSH, LH, FSH, Prolactin, GH, ACTH - Cortisol.
I suggest that instead of getting all the hormones done, it would be better and cheaper to consult a hormone specialist (Endocrinologist) who can decide which tests are really necessary.