Thyroxine treatment facilitates prolactin secretion and induces a state of photorefractoriness in thyroidectomized starlings.

Sexually immature male starlings were radiothyroidectomized while held under short day-lengths. They were then subjected to long (18-h) photoperiods and the testes developed rapidly and apparently normally to full maturity. As expected, thyroidectomy prevented an onset of photorefractoriness and after 140 days the testes were still fully developed (testicular width 6.5 +/- 0.8 mm). Half of the birds were then given thyroxine (0.011 mmol/l) in the drinking water for 14 days while the others were maintained as controls. At the end of this time the birds still had fully developed gonads (testicular width 7.8 +/- 0.3 mm) but after a further 14 days testicular regression began in the thyroxine-treated birds. The testes of all individuals in this group were fully regressed (width 1.8 +/- 0.1 mm) by 56 days after the end of thyroxine administration and moulting of the flight feathers had begun. No recrudescence of the gonads was subsequently noted in the next 2 months and the birds were apparently photorefractory. The short period of thyroxine treatment also caused a rapid and prolonged increase in plasma prolactin levels from 2.0 +/- 0.3 to 16.8 +/- 2.6 micrograms/l. No testicular regression or moulting was observed in the control birds and their plasma prolactin levels remained very low (below 4.0 micrograms/l) throughout the experiment. In thyroidectomized and castrated starlings held on 18-h daylengths, 14 daily injections of thyroxine (100 micrograms/bird per day) caused a rapid and permanent decrease in circulating FSH to basal levels (reached about 36 days after thyroxine treatment began, at which time the birds moulted). The treatment also caused a marked increase in plasma prolactin lasting for about 50 days.