Two experiments were conducted to test whether thyroid hormones are required for the cessation of the breeding season in female red deer. In Expt 1, 16 mature hinds were allocated early in the breeding season (May) to the following groups: thyroidectomized (THX), thyroidectomized and treated with subcutaneous thyroxine implants which increased plasma tri-iodothyronine to physiological concentrations (THX + T4), or euthyroid controls. Plasma progesterone concentrations consistent with oestrous cyclicity were evident in all hinds during the breeding season (May–September) but after 23 September plasma progesterone concentrations became generally low (< 2.0 nmol l−1) in control and THX + T4 hinds. In contrast, thyroidectomized hinds not receiving thyroxine continued to exhibit circulating progesterone comparable with breeding season concentrations until the end of the experiment (late December). Nine hinds were ovariectomized or ovariectomized and thyroidectomized in May and treated with subcutaneous oestradiol-impregnated capsules, which were removed for periods of about 1 month during August, November and January, to test whether thyroid hormones are involved in the seasonal decline of LH secretion (Expt 2). In the presence of oestradiol, basal LH concentrations and episodic LH secretion were low during the non-breeding season (September–March) (P < 0.05). During this season, basal and GnRH-induced LH concentrations and LH pulse frequency remained similar for both groups except in the absence of oestradiol, when basal LH (3.1 ± 0.8 versus 1.3 ± 0.5 ng ml−1, P < 0.05 and GnRH-induced LH (25.9 ± 3.6 versus 4.7 ± 0.4 ng ml−1, P < 0.001) concentrations were higher in thyroidectomized hinds than in euthyroid hinds, respectively. Twenty-five hinds were ovariectomized and thyroidectomized during the breeding season and treated with subcutaneous thyroxine implants at different times to identify when thyroid hormones are required to be present to suppress LH concentrations during the non-breeding season in the absence of oestradiol. Thyroxine treatment at the beginning of or during the non-breeding season was effective in suppressing plasma LH concentration, but this action of thyroid hormones did not occur during the breeding season. These results indicate that, in red deer hinds thyroxine is required for termination of the breeding season and that thyroid gland secretions specifically block steroid-independent inhibition of reproductive activity during the non-breeding season. This inhibitory mechanism requires thyroid hormones to be present only from around the time of the end of the breeding season for their normal expression, and they remain responsive to thyroid hormones after this period.
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