Thyroid dysfunction and thyroxine-dependent programming of photoinduced ovarian growth in American tree sparrows...

The time course of thyroid dysfunction after injecting 0.30 mCi Na131I was charted in female American tree sparrows (Spizella arborea) transferred to long days on Day 0 and challenged with 30 micrograms bovine thyroid-stimulating hormone on Postinjection Days 4, 7, 46, and 105. Serum T4 (L-thyroxine) was not detectable in thyroidectomized birds, indicating complete thyroid dysfunction by Day 4 and no restoration of thyroid function by Day 105. By contrast, serum T4 concentrations in similarly challenged thyroid-intact controls greatly exceeded assay sensitivity. To determine whether a single injection of T4 can program thyroidectomized female tree sparrows for seasonal reproduction and postnuptial molt, as it can thyroidectomized male tree sparrows, thyroidectomized females were injected with T4 (100 micrograms) or alkaline vehicle (V) either on the first day of photostimulation or 1 week before photostimulation (during which time injected T4 likely was cleared metabolically). Females injected with T4 on the first day of photostimulation showed robust ovarian growth similar to that reported for photostimulated euthyroid females. Both groups of V-injected females, as well as females injected with T4 1 week before photostimulation, showed slow, but reliable, ovarian growth, indicating that thyroidectomized birds can detect an increase in day length. When birds were moved at Week 7.5 or 9 to constant light and given T4 in drinking water (a qualitative assay for absolute photorefractoriness), only one tested photorefractory and molted. The remaining birds, including those injected with T4 1 week before photostimulation and both groups of V-injected controls, tested photosensitive and did not molt. Taken together, these results indicate that T4, or one of its metabolites, programs thyroidectomized female tree sparrows for ovarian growth early during photostimulation. A single T4 injection (100 micrograms) on the first day of photostimulation usually also programs thyroidectomized male tree sparrows for photorefractoriness and molt. Such an injection is far less effective in programming females for these two late-season events.