The thyroid and the hypothalamus-pituitary-ovarian axis in American tree sparrows (Spizella arborea)

Prompted by evidence of a linkage between the thyroid gland and seasonal reproduction in obligately photoperiodic American tree sparrows (Spizella arborea), we measured circulating T4 (L-thyroxine), hypothalamic cGnRH-I (chicken gonadotropin-releasing hormone, variant I), pituitary and plasma LH (luteinizing hormone), and ovarian mass in euthyroid females moved from short to long days. Our purpose was to correlate temporal changes in T4 and cGnRH-I with each other and with pituitary and plasma LH and ovarian mass. T4 increased early during photostimulation and peaked at Week 1 or 2 on 20L:4D (20 hr light:4 hr dark per day). Hypothalamic cGnRH-I also increased during photostimulation and peaked between Weeks 2 and 4. Pituitary and plasma LH peaked at Weeks 4 and 1, respectively, and ovarian mass increased significantly by Week 6. In a separate study, we charted the progression of postnuptial molt. Molt of the primary remiges began at Week 9, proceeded rapidly through Week 15, and was complete by Week 18. In view of a demonstrated role for the thyroid gland in the control of seasonal reproduction in female tree sparrows and the time-dependent effects of thyroidectomy after photostimulation in male tree sparrows, the observation that circulating T4 increases early during photostimulation suggests the possibility of a causal relationship between the thyroid gland and the hypothalamus-pituitary-ovarian axis, hence a thyroid-hypothalamus-pituitary-ovarian axis.