Effect of L-thyroxine on ovarian development and gestation in the viviparous guppy, Poecilia reticulata.

Treatment of female guppies from 1 day postpartum with L-thyroxine-sodium salt (Eltroxin, Glaxo) by immersion (0.5 ppm in 25% seawater renewed daily) significantly shortened the brood interval without affecting the brood size or the weight of the brood. This suggests an acceleration of early development of the fish by thyroxine. The thyroxine treatment also accelerated the development of the next batch of oocytes, particularly exogenous vitellogenesis (yolk-globule stages). The evidence suggests a role of the thyroid in vitellogenesis and gestation in the guppy.