Peripheral Metabolism of Thyroid Hormones in Vitamin A-Deficient Rats

Abstract

Some aspects of thyroid hormone metabolism were studied in rats on a vitamin A-deficient diet for 7–8 weeks. It was shown that, in deficiency, thyroxinemia, the extrathyroidal thyroxine (T₄) pool and the T₄ biological half-life were increased, and the T₄ distribution space and the T₄ disposal rate were similar to those in control rats. The in vitro formation of triiodothyronine (T₃) from T₄ by liver microsomes was slightly decreased under basal conditions but similar after addition of glutathione. So the increased T₄ half-life in deficient animals may be related to a decreased 5’-monodeiodase activity resulting from decreased glutathione availability. In deficient rats, the T₃ serum level was increased, the T₃ biological half-life was similar and the T₃ distribution space was much decreased.

Effect of Concurrent Vitamin A and Iodine Deficiencies on the Thyroid-Pituitary Axis in Rats

“…In iodine-sufficient animals, severe VAD alone causes thyroid hypertrophy (9,24), reduces thyroidal iodine uptake (25), impairs synthesis of Tg and coupling of iodotyrosine residues to form thyroid hormone (8), and decreases intrathyroidal T 3 and T 4 (8,9). In the periphery, severe VAD increases total and free T 3 and T 4 (8,10), reduces hepatic conversion of T 4 to T 3 (8,26), and decreases T 3 uptake and binding (11,12). Because we did not measure deiodinase expression and/or activity in the thyroid or the liver, this study does not provide data on these potential mechanisms.…”

Thyroid Hormones in Vitamin A-Deficient Rats: Effect of Retinoic Acid Supplementation

“…It is only possible to suggest some hypoth eses supported by results already published [15] concerning vitamin A deficiency in rats: (i) the high levels of thyroxinemia in vitamin A-deficient rats are expected as a result of a reduced peripheral metabolism of T4, and (ii) the high levels of serum T3 are expected as a result of a smaller penetration of this hormone into the target cells.…”

Effects of a Large Dose of Retinol or Retinoic Acid on the Thyroid Hormones in the Rat

“…Semisynthetic diet with 18% casein and containing 7.5 pg retinol as retinol palmitate (vi tamin A hydrosoluble, Hoffmann-La Roche, France) per gram dry diet [3,[5][6][7], Retinol Diet. Control diet plus 375 pg retinol as retinol palmitate (vitamin A hydrosoluble, HoffmanLa Roche, France) per gram dry diet; such a dose pro vides a daily food intake of approximately 6,750 pg of retinol (22,500 IU).…”

“…This experiment utilized the method described previously [7]. After administration of radioactive hormones, the analysis of the disappearance of serum radioactivity allows determination of some of the usual parameters of the peripheral turnover: TDS = T4 or T 3 apparent distribution space which is the virtual space in which the tracer dose of hormone would be found if it was distributed at a concentration equal to that of serum; ETP = extrathyroidal T4 or T3 pool which is the quantity of exchangeable T4 or T 3 within the organism (ETP: TDS X TT4 or TT3).…”

“…The fact that a retinol-defi cient diet causes various changes in the thy roidal status [3][4][5][6][7] brings into question the status of the thyroid hormones when large doses of retinoids are administered, but only little was known about the effects of large doses of retinol [8,9], and virtually nothing is known about the effects of large doses of other retinoids. We have therefore studied in rats the effects of a large dose of retinol or retinoic acid on the serum level and the turn over of thyroxine (T4) and triiodothyronine (T3), and the partition of l25I-T3 between tis sues and serum.…”