Peripheral tissue mechanism for maintenance of serum triiodothyronine values in a thyroxine-deficient state in man.

Abstract

As bstract. The present study was undertaken to define the source of endogenous triiodothyronine (T3) production responsible for maintaining serum T3 levels in euthyroid subjects with depressed serum thyroxine (T4) values. After withdrawal from 4 wk of exogenous T3 administration, a 22% decline in serum T3 values (from 129±6 to 99±4 ng/dl) was observed in six euthyroid subjects, despite a twofold reduction in serum T4 concentrations (from 7.5±0.5 to 3.2±0.5 ug/dl). This was accompanied by a nearly twofold increase in serum T3/T4 ratio values (17±1 to 29±6) but no significant alteration in reverse T3/T4 ratio values. This phenomenon did not appear to be thyroid stimulating hormone (TSH) dependent, since base-line serum TSH values were subnormal. Nor was it dependent on changes in thyroid gland function, since a blunted T3 response to exogenous bovine TSH occurred and pharmacologic doses ofiodide did not influence the phenomenon. The finding in three athyreotic subjects that serum T3/T4 ratio values increased from 14±1 on T4 therapy (mean serum T4, 9.6±0.8 ug/dl and T3, 132±8 ng/dl) to 40±2 after withdrawal from 2 wk of T3 administration (serum T4 1.2±0.1 ttg/dl and T3 46±3 ng/dl) provided direct evidence that an alteration in peripheral thyroid hormone metabolism was probably responsible for these findings previously observed in euthyroid subjects. The results of this study support the possible existence in euthyroid man of a peripheral tissue autoregulatory mechanism for maintaining serum T3 values in states of T4 deficiency. Whether this process

Detection and quantification of 3,5,3′-triiodothyronine and 3,3′,5′-triiodothyronine by electrospray ionization tandem mass spectrometry

“…T he thyroid hormones, 3,5,3=,5=-tetraiodothyronine (T4), 3,5,3=-triiodothyronine (T3), and 3,3=,5=-triiodothyronine (rT3; reverse T3) (Figure 1), are important in regulating a number of biological processes, including growth and development, carbohydrate metabolism, oxygen consumption, protein synthesis, and fetal neurodevelopment [1]. T4 is secreted by the thyroid gland.…”

Isomeric discrimination and quantification of thyroid hormones, T3 and rT3, by the single ratio kinetic method using electrospray ionization mass spectrometry

“…T 4 is an important source of the T 3 and rT 3 , as it is responsible for 80% of the whole amount. The two hormones secreted by the thyroid gland are produced by peripheral monodeiodination [1] of T 4 , but alternative routes of enzymatic deiodination can also produce either T 3 or rT 3 [2]. The process involves the removal of an iodine atom from carbons 5= and 5 on the outer ring of T 4 , respectively (Scheme 1).…”

“…T 3 is metabolically active and important in regulating biological processes, including growth and development, body temperature, heart rate, oxygen consumption, protein synthesis, and fetal neurodevelopment [3]. Therefore, it is used to treat hypothyroidism or thyroid hormone deficiency in medical practices.…”

“…Under normal conditions, 45% to 50% of the daily production of T 4 is transformed into rT 3 [6]. In some instances, there may be preferential formation of rT 3 to reduce energy output, that is, low T 3 syndrome.…”

“…Under normal conditions, 45% to 50% of the daily production of T 4 is transformed into rT 3 [6]. In some instances, there may be preferential formation of rT 3 to reduce energy output, that is, low T 3 syndrome. In addition, rT 3 may inhibit deiodinases, thereby directly interfering with the generation of T 3 from T 4 , or with subsequent breakdown of T 3 [5,7,8].…”