Are you sure you understand how this works? Typically, a TSH is drawn and if that's out of range, an FT4 will be drawn (and sometimes an FT3).
Thyroid is counter-intuitive in that when one is hypo, TSH is high and actual thyroid levels (Free T4 and Free T3) are low. That's because the pituitary recognizes the low thyroid hormones, so it produces TSH to stimulate the thyroid into producing more hormones. The lower the thyroid hormones go, the higher the TSH goes.
On the newborn screen, a TSH was done (as usual) and was WNL, so no further testing was done. On the latest test, the TSH was too high and that triggered the Free T4.
What's the reference range for the FT4? Ranges vary lab to lab and have to come from the individual's report. Not having that reference range makes it hard to say much, but from what I've seen of reference ranges, if my son had an FT4 of 0.76, I'd be demanding a Free T3 test and probably hormone replacement medication.... or a second opinion at the least. You might ask for referral to a pediatric endocrinologist.