A radioimmunoassay technique for measuring the thyrotrophin (TSH) concentration of sheep and cattle plasma is described. The sensitivity of the assay allowed the measurement of 1–50 ng TSH/ml unextracted plasma. Cross-reaction with ovine luteinizing hormone, prolactin and growth hormone was very low. The average recovery of added TSH was 103 ± 4·1 (s.e.m.)% and the between-assay coefficient of variation was 13·8%.
The normal plasma TSH levels of sheep and cattle were approximately 2·5 ng/ml (5 mu. bovine TSH/100 ml). Foetal sheep had plasma TSH concentrations of approximately 3·2 ng/ml during the last 20 days of gestation. Levels of TSH in the circulation decreased abruptly after hypophysectomy of the foetal lamb and a decline in the plasma thyroxine (T4) concentrations was also apparent within 24 h of the operation. However, thyroidectomy of adult and foetal sheep did not increase plasma TSH concentrations until almost all the T4 had been cleared from the circulation.
The injection of T4 into thyroidectomized sheep rapidly reduced plasma TSH concentrations to normal values. However, the continued injection of T4 did not further reduce TSH concentration. The injection of T4 or triiodothyronine into normal sheep was also without effect on plasma TSH concentrations.
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Online ISSN: 1479-6805
Print ISSN: 0022-0795
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