Thyroid function studies in normal and cold-exposed rabbits using 125I.

Carlson, L. D., H. V. Roohk and O. Wilson. Thyroid function studies in normal and cold-exposed rabbits using 125I. Acta physiol. scand. 1973. 89. 359–373. Normal uptake and retention of 125I in thyroid was measured for 2–17 weeks in rabbits, and the effect of TSH, T4, acute and extended cold exposure was studied. There were marked interindividual variations in maximum uptake (mean 7 %, range 2.5–18 %), in time to peak (12–72 h), in the slope of the retention curve, and in changes in release rate. Repeated doses gave different maximum uptakes, but time to peak and form of retention curve remained essentially the same for each animal. The mean biological half-life for a 35-day period was 12 days for 8 rabbits; the initial half-life of around 9 days for the 1st week gradually increased to around 16 days during the last 3 weeks. This change suggests a shift of the labelled iodine into thyroidal compartments with slower turnover rates. Analysis of the multiexponential slope showed that the release phases for all rabbit were satisfactorily represented operationally by two exponentials. The mean values for these corresponded to biological half-lives of 6.4 and 21.1 days. Neither acute and more severe (-3o and 6o C for 50 h) nor moderate, extended cold (9–10o C for 7–16 days) had the slightest influence on the loss of radioiodine from the thyroid, which nevertheless responded to small doses of TSH.

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