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Abstract
Thyroid hormone levels before and after thyrotropin releasing hormone (TRH) injection were compared at four different stages in relation to parturition in nine heifers. Basal thyroxine (T4) levels were similar before conception and two weeks before calving, but the T4 responses to TRH were reduced near parturition. Hormone levels and responses were low 2 weeks as well as three months after parturition. Both basal levels of T3 and T3 responses to TRH were low near parturition. Three months after parturition both basal level and TRH responses were comparable to those observed before conception. Low basal levels of thyroid hormones near parturition were not associated with an increased disappearance rate. Reduced thyroid secretion activity during early lactation must be the main explanation for the low thyroid hormone levels seen at this stage.