Thyroid hormone levels change markedly during the periparturient period. In order to clarify their postpartum recovery times, changes in serum thyroxine (T
4), serum triiodothyronine (T
3), serum nonesterified fatty acid (NEFA), and plasma glucose levels were measured in 6 clinically healthy Holstein cows. The measurements were carried out over a period of 3 months in the Tokachi District of Hokkaido. Serum T
4 and T
4/T
3ratio started decreasing 2 weeks before parturition and reached minimum values (0.9μg/d
land 13.1, respectively) 3 days after parturition. Levels recovered gradually thereafter. Serum T
3 (49ng/d
l) reached the minimum days after parturition and plasma glucose levels (38.6mg/d
l) 3 days after parturition. Thereafter, those levels also increased gradually. Serum NEFA levels started to increase 3 weeks before parturition; thereafter they decreased. These findings show that in healthy cows it takes about 2 postpartum months for thyroid hormone levels to recover.
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