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Biomedical subjects

H van Toor

Publications and source records attributed to H van Toor.

24 records · Page 2Linked to original sources
PubMed1992

Effect of starvation and subsequent refeeding on thyroid function and release of hypothalamic thyrotropin-releasing hormone.

Effects of starvation on thyroid function were studied in 5- to 6-week-old (R x U) F1 rats. Starvation lowered plasma TSH in female, but not in male rats. Plasma T4 and T3 levels decreased, whereas the dialysable T4 fraction increased during starvation. Free T4 (FT4) levels decreased rapidly in females, but only after prolonged fasting in male rats. Glucose decreased, and free fatty acid levels increased during starvation. Peripheral TRH levels did not change during food deprivation. Since effects of starvation were most apparent in young female rats, such rats were used to study hypothalamic TRH release during starvation and subsequent refeeding. Basal in vitro hypothalamic TRH secretion was less in starved rats than in control or refed animals. In vitro hypothalamic TRH release in medium with 56 mM KCl increased 3-fold compared to basal release, and in these depolarization conditions TRH release was similar between hypothalami from control, starved and refed rats. In rats starved for 2 days, TRH level in hypophysial portal blood was lower than that of controls. Thus, diminished thyroid function during starvation may at least in part be caused by a reduced hypothalamic TRH release.

Animals↗
PubMed1988

Three-compartmental analysis of effects of D-propranolol on thyroid hormone kinetics.

Tracer thyroxine (T4), 3.3',5-triiodothyronine (T3), and 3,3',5'-triiodothyronine (rT3) kinetic studies were performed in normal T4 substituted subjects before and during oral D-propranolol treatment to determine whether changes in thyroid hormone metabolism in a propranolol-induced low-T3 syndrome result from inhibition of 5'-deiodination or inhibition of transport of iodothyronines into tissues. Data were analyzed according to a three-compartmental model of distribution and metabolism. T4 plasma appearance rate decreased by 16% (P less than 0.01), reflecting a decreased intestinal absorption of orally administered T4 during propranolol. Serum T4 and free T4 levels increased significantly by 14%, whereas T4 metabolic clearance rate (MCR) was lowered by 26% (P less than 0.001). No changes were observed in size of the three T4 compartments or in fractional and mass transfer rates of T4 from plasma to the rapidly (REP) and slowly (SEP) equilibrating pools. Serum T3, free T3, T3 plasma pool, T3 mass transfer rate to REP and SEP, and the T3 pool masses were all significantly decreased during propranolol to a similar extent as the T3 plasma production rate (PR). T3 MCR decreased by 14% (P less than 0.05). Serum total and free rT3 increased, whereas the rT3 MCR was substantially lowered during propranolol (P less than 0.001). The rT3 plasma pool, rT3 REP and SEP, and the mass transfer rates to REP and SEP increased, whereas no alterations were observed in rT3 PR and fractional transfer rates of rT3 to REP and SEP.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1986

Effects of caloric deprivation on thyroid hormone tissue uptake and generation of low-T3 syndrome.

Changes in thyroid hormone metabolism in the low-3,5,3'-triiodothyronine (T3) syndrome cannot be fully explained in all conditions by a decrease in 5'-deiodinase activity. Recent observations showed that in rat hepatocytes iodothyronines are taken up by an active transport mechanism. To investigate whether regulation, i.e., inhibition of active transmembraneous transport for iodothyronines in humans may contribute to the generation of the low-T3 syndrome, tracer thyroxine (T4) and T3 kinetic studies were performed in 10 obese subjects before and after 7 days on a 240 kcal diet. Kinetics analyses were performed according to a three-pool model of distribution and metabolism for both T4 and T3. For T4 kinetics, during caloric deprivation serum total T4 and plasma pool did not change and production rate and metabolic clearance rate (MCR) were significantly lower. Despite a significantly higher serum free T4, the mass transfer rate to the rapidly equilibrating pool (REP) and the slowly equilibrating pool (SEP) diminished significantly, leading to smaller tissue pools. For T3 kinetics, both serum total T3, free T3, plasma pool, and production rate diminished significantly, while MCR remained unchanged. Mass transfer rates to the REP and the SEP were lowered by approximately 50%, leading to smaller tissue pools. These changes cannot be fully explained by a similar decrease of serum free T3 (only 25%), indicating a diminished transport efficiency for T3. In conclusion, during caloric restriction, transport of T4 and T3 into tissues is diminished, and this phenomenon is much more pronounced for T4 than for T3.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗