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TH peak occurs after birth in a period of molt that precedes the weaning
period (
Atkinson, Amould, & Mashburn, 2011
). This suggests that the pre-
cise timing of the TH peak can vary from one species to another and that it
would be very interesting to study more systematically these levels in several
mammalian orders to see if a clear pattern emerges and whether it is related
to specific developmental strategies.
Another rodent species, the guinea pig (
Cavia porcellus
), seems partic-
ularly interesting in thisrespect.Thisspecieshasaverylonggestationpe-
riod as compared to the mouse, the level of free T4 in the serum doubles
between 45 and 62 days of gestation, reaching a maximum near term (at
63-68 days of gestation). However, the levels of total or free T3 in serum
do not follow the same path, whereas the levels of reverse T3 increase as
those of T4 (
Castro, Alex, Young, Braveman, & Emerson, 1986
). This
suggests that a significant proportion of the T4 is degraded into rT3 in
the serum. Whether or not this corresponds to an increase of T3 concen-
tration within tissues remains to be studied. What is clear from these data,
however, is that a high level of T4 occurs during postembryonic develop-
ment in the mouse whereas in guinea pig, such high levels are reached at
the end of gestation. It is not clear, however, how the level of T4 and T3
change during postembryonic life in guinea pig. However, the authors of
this study conclude that, in contrast to mouse and rat, maturation of the
thyroid system occurs
in utero
in guinea pig (
Castro et al., 1986
). Taken
together, the data coming from mammals show that there is a TH peak
during late gestation or early postembryonic life, but that the precise
timing of this peak, relative to birth, may change from one species to
another.
5.2. Role of TH in late gestation and early postembryonic life:
data from mice
In mammals, during weaning, the pup becomes independent from the
mother, ceasing to suckle. This transition requires physiological modifica-
tion and remodeling of several organs among which the intestine develops
significantly during this period. Changes in other organs are also observed:
bone ossification acceleration, appearance of autonomous thermoregula-
tion, and brain remodeling (for review, see
Forrest & Vennstr¨m,
2000
). These events are dependent on a functional TH signaling pathway.
Indeed, analysis of Pax8 KOmice, that are severely hypothyroid due to the
failure of the thyroid gland to develop, confirms this notion: these mice
