Abstract
Abstract Daily patterns of plasma corticosterone (B) and prolactin (PRL) concentration were measured in female rats which were intact (INTACT), thyroidectomized (THYREX), or thyroidectomized and given thyroxine (T 4 ) replacement (THYREX + T 4 ) (20 μg T 4 /day). Bimodal daily rhythms of plasma B were present in INTACT rats and THYREX + T 4 rats. However, no plasma B rhythm was detectable in THYREX rats. THYREX + T 4 rats, which had greater mean T 4 concentrations than INTACT rats (6.0 μg/dl vs. 3.5 μg/dl), had a plasma B rhythm of greater amplitude than INTACT rats. Plasma PRL rhythms were detectable in INTACT and THYREX + T 4 rats, but not in THYREX rats. INTACT rats had a single peak in plasma PRL, whereas three plasma PRL peaks were present in THYREX + T 4 rats. It was concluded that thyroid hormones are required for the expression of plasma B and PRL rhythms and that the levels of thyroid hormones can alter the amplitude of the B rhythm and the shape of the PRL rhythm.
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