Synthroid is the natural hormone produced by the thyroid gland.  It does
appear in the milk, but then thyroxin *normally* appears in the milk.
The amount that would be added to the milk if the mother is taking the
medication orally is probably close to nil, as presumably, the mother
would *usually* be taking it because she is not producing enough
herself.  Even if she takes the thyroid for suppression of a hot nodule
(uncommon), the amount the baby received would be insignificant.
Furthermore, if the baby is normal, even if he received *too much*, a
possibility I refuse to admit, his own pituitary-thyroid axis would
decrease stimulus of his thyroid, so that everything would be normal.

Only if the mother gets into the toxic range is is a possibility that
the thyroid hormone would decrease the milk supply.  It is possible, if
the mother is hypothyroid, that TSH would be quite high.  TSH has
actions on the breast similar to prolactin, so taking thyroid hormone
might decrease the TSH level in the blood.  But this is theoretical, and
seems far fetched to me.

Jack Newman, MD, FRCPC