Don't really know about newborns or pediatric populations and we really
don't have normals on our reports.

For adults this is typical.

"TSH values greater than 20 mU/L, even in the absence of overt clinical
signs of hypothyroidism, mean primary hypothyroidism. Values less than that
and in particular less than 10 mU/L in asymptomatic patients are more
difficult to evaluate. It is important to become familiar with the reasons
for increases and decreases in serum TSH in the absence of thyroid
dysfunction. Nonthyroidal illnesses can account for both increases and
decreases."

Although the stated reference range for adults can be as high as 5 mU/L some
endocrinologists consider values greater than 3 as borderline.

The free T4 is important in trying to determine it's relationship to the
TSH. A decrease Free T4 would indicate primary hypothyroidism. A normal free
T4 would be borderline or Subclinical Hypothyroidism and an increase in free
T4 would indicate a TSH secreting tumor or thyroid hormone resistance.

Although not in play with identical twins, fraternal twins have been known
to exhibit chimerism of tissue sharing. They may contain DNA from two
individuals within one person.
Although strictly conjecture, hypothyroidism and fetal-maternal chimera has
been postulated as a cause for autoimmune hypothyroidism.