CLINICAL CHEMISTRY OF GRAIN-FED CATTLE.: III. PLASMA THYROID PROFILES WITH NOTES ON INTERFERENCE BY EDDI AND HYPOTHYROID VALUES IN HEREFORDS

Linear thyroid profiles consisting of total plasma I (TI), protein-bound iodine (PBI), thyroxine iodine (T 4 I), tri-iodothyronine uptake (T 3 U) and the corresponding indices of T 7 I = T 4 I* T 3 U/100 and thyroid binding globulin estimate TBGE = T 3 U −1  + T 4 r −0.25 were prepared for 15 grain-fed (test) and 15 hay-fed (control) Hereford steers over a 186-day feeding period. The grain diet, which contained 20 mg/kg of I 2 as ethylenediaminedihydriodide (EDDI), consisted of 90% steam-rolled barley, 5% beet pulp pellets and 5% protein–mineral concentrate, supplemented with local alfalfa hay fed at the rate of 0.9 kg/head/day. The control diet was local alfalfa hay given ad libitum. CoI salt blocks containing 0.028% Ca (IO 3 ) 2 were available to both groups, free choice. In the test group, TI increased threefold immediately upon introduction of the grain diet, and remained elevated. Plasma PBI increased from 5.0 to 12 μg/dl on day 50 and then fell slowly to 9.0 μg/dl by day 180. T 4 I, T 7 I and TBGE decreased during the first 30 days and returned to the original values of 4.6 ± 1.2, 2.3 ± 0.8 and 1.48 ± 0.11 μgI/dl. T 3 U decreased from 51 to 48% over the feeding period. The PBI and T 4 I values were not related, probably because EDDI or a metabolite of it interferes with the PBI test. In the control group, the profile values at the beginning and end of the feeding period were in μgI/dl: TI, 7.3 ± 1.5 and 5.6 ± 1.0; PBI, 5.9 ± 1.0 and 5.3 ± 0.7; T 4 I, 4.2 ± 0.4 and 2.8 ± 0.5; T 3 U%, 55.8 ± 8.0 and 54.4 ± 6.9. These values provided corresponding index values of T 7 I, 2.3 ± 0.4 and 1.5 ± 0.4 and TBGE, 1.44 ± 0.04 and 1.31 ± 0.06. The control diet did not provide enough available I 2 ; the thyroid profiles showed a trend towards hypothyroidism, and histologic examination post-mortem revealed thyroid hyperplasia.