Figure 1.

Circulating triiodothyronine (T3) and reverse T3 (rT3) levels in normal fetal sheep (Polk 1995) (A). The prepartum surge is driven by the prepartum cortisol surge stimulating deiodinase expression and conversion from T4. Cortisol stimulates D1 activity and suppresses D3 activity. Representation of the normal timing and changes in binucleation of fetal sheep left ventricular cardiomyocytes (B). Binucleation is an index of terminal differentiation, after which cells exit the cell cycle and no longer divide. In the fetal sheep heart this process begins just after 100 days gestational age and is ~75% binucleated at the time of birth. The dashed line at 125 days gestational age represents the timing of studies discussed in this review (Polk 1995, Jonker et al., 2007 and Chattergoon et al., 2012).