Thyroxine: convesion to triiodothyronine by isolated perfused rat heart

Triiodothyronine-induced hyperthyroidism and reproduction in female rats

The effect of L-triiodothyronine (L-T3)-induced hyperthyroidism on reproduction in female rats was studied. Treatment groups used were normal control, 1, 2, 4, 8, and 16 micrograms L-T3/100 g body weight (b.w.)/day. Subcutaneous injections of L-T3 began the first day of estrus until day 1 or day 19 of lactation. All rats given 1, 2, or 4 micrograms L-T3 had regular estrous cycles and normal number of pregnancies. Levels of L-T3 at 8 or 16 micrograms results in abnormal estrous cycles. Only one pregnancy occurred in the 8 micrograms group and no rats became pregnant in the 16 micrograms group. While groups on 1 or 2 micrograms L-T3 had normal parturition, groups on 4 micrograms or more showed signs of dystocia. Only the 1 microgram L-T3 group lactated to day 20, but at 67% of normal control. All levels above 1 micrograms showed loss of pups by day 2 of lactation due to lack of milk production, loss of maternal behavior, and cannibalism. Mammary gland wet and dried fat-free tissue (DFFT) weights were decreased in all treated groups except 1 microgram L-T3. The wet and DFFT weights of thyroids and uteri on days 2 or 20 of lactation were not altered by L-T3 treatment, but levels above 1 microgram increased both wet and DFFT weights of adrenal glands and ovaries. The data indicate that the no observed adverse effect level (NOAEL) of L-T3 for female rat reproduction would be below 1 microgram/100 g b.w./day. L-T3 treatment at 1 microgram/100 b.w./day or above is detrimental. These effects may be mediated through a hormonal imbalance due to the accelerated metabolic rate in reproductive tissues associated with hyperthyroidism.

Thyroid indices in arterial and venous cord blood: significantly greater levels of reverse triidothyronine in venous blood than in arterial blood

Paired arterial and venous cord blood samples were obtained from 42 normal newborns (24 males and 18 females). T4 was determined in all paired samples. In addition, other indices were determined: T3 in 40, RT3 in 29, TBG in 29, thyroglobulin (Tg) in 14, and TSH in 11. Gender difference in any of the thyroid indices was not found. Arterial and venous cord serum thyroid indices correlated positively (T4, r = 0.743; T3, r = .907, rT3, r = .920; TBG, r = .752; Tg, r = .934, and TSH, r = .989; P less than 0.005). The difference between the means +/- SD of arterial and venous levels was significant (P less than 0.01) only for rT3 (191 +/- 43.2 v 224 +/- 55.8 ng/dL). Arterial (T4 v T3, r = .453, P less than .005; T4 v RT3, r = .660, P less than 0.005) and venous (T4 v T3, r = .620, P less than 0.005; T4 v rT3, r = .612, P less than 0.005); T3 and rT3 levels correlated positively with T4 levels. In contrast, T3 and rT3 levels for arterial (r = .216, P greater than 0.1) and venous (r = .216, P greater than 0.1) samples did not show a significant correlation. These data are in keeping with earlier reports for animal placental models studied in vitro, suggesting that placental inner ring deiodination of maternal thyroxine is a source of fetal RT3.

The metabolism of T4 and T3 in cultured chick-embryo heart cells

Thyroxine (T4) and tri-iodothyronine (T3) were metabolized in cultured chick-embryo heart cells via inner-ring de-iodination and O-sulfation. The products of T3 metabolism were 3,3'T2, 3'T1, and sulfated esters of T3 and 3,3'T2. The major product of T4 degradation was 3,3',5'-T3 (rT3). ONo T3 was produced from T4. Propylthioracil inhibited the metabolism of T3. Pretreatment of cultures with T3 or T4 enhanced the metabolism of both hormones; actinomycin D and cycloheximide inhibited the stimulatory effect of T3. The stimulation by T3 was linearly related to the log of the concentration of T3 in cells grown in normal chick serum or in cells grown in dehormonized serum. These results suggest that thyroid hormones induced an increased synthesis of the enzymes involved in their metabolism and therefore may regulate their own disposal rate.

Effect of starvation, nutriment replacement, and hypothyroidism on in vitro hepatic T4 to T3 conversion in the rat

To evaluate the effect of starvation, oral and i.v. nutriments, and hypothyroidism on the peripheral conversion of thyroxine (T4) to 3,3', 5-triiodothyronine (T3) in the rat and mouse, an in vitro system for assessing T4 conversion to T3 by fresh liver homogenates was used. A 2-day starvation in the rat reduced hepatic T3 generation from T4 by 47% +/- 3.5% (mean +/- SE) in six separate experiments and also impaired the metabolism of 125I-r-T3. Administration of carbohydrate (CHO) and amino acids (P), but not lipid (L), significantly increased T3 generation above values observed in the starved rat. The mean serum glucose concentration was similar in all nutriment-infused groups, but serum insulin was significantly greater in the CHO- and P-infused as compared to the L-infused rats. These findings suggest that CHO and P, but not L, are important modulators of hepatic outer ring thyronine deiodination in the rat, perhaps due to increased intracellular glucose. Hypothyroidism in the rat induced by thyroidectomy and congenital secondary hypothyroidism in the dwarf mouse resulted in a striking decrease in hepatic conversion of T4 to T3. This decrease was restored to normal by the daily s.c. administration of physiologic doses of T4 (1.5 microgram/100 g) or T3 (0.5 microgram/100 g) for 14 days, and was increased above normal following treatment of normal rate with greater than physiologic doses of T4 (3microgram/100 g) or T3 (1 microgram/100g). In vitro hepatic conversion of T4 to T3 is, therefore, dependent upon thyroid function. Since 2-days starvation in the rat was associated with decreased serum concentrations of T4, T3, and TSH, and hypothyroidism resulted in decreased conversion of T4 to T3, the effect of a constant 2-day infusion of physiologic doses of T4 or T3 in the starved rat on the in vitro deiodination of T4 was assessed. Thyroid hormone replacement did not enhance the conversion of T4 to T3 in the starved rat. These observations suggest that the starvation-induced decrease in hepatic generation of T3 from T4 is not due to hypothyroidism and that the mechanism(s) of the decreased T3 production observed in starvation and hypothyroidism is different.

Reduced peripheral conversion of thyroxine to triiodothyronine in patients with hepatic cirrhosis

The role of liver in the peripheral conversion of thyroxine (T4) to triiodothyronine (T3) was studied in normal subjects and patients with alcoholic liver disease by measurement of thyrotrophin (TSH) and total and free T4 and T3 in randomand serial serum samples. Also, T4 to T3 conversion rates and T3 disposal rates were compared by noncompartmental analysis. While the mean total serum T4 values were similar for the two groups, 8.6 and 8.1 mug/kl, the mean free T4 value was significantly higher in the cirrhotic patients (3.3 ng/dl) than in the normal subjects (2.1 ng/dl, P less than 0.001). The mean serum T3 value, 85 ng/dl, was significantly reduced in the hepatic patients as compared to a mean serum T3 value of 126 ng/dl in the normal subjects (P less than 0.001), while the free T3 value was 0.28 ng/dl in both groups. The reduction of the serum total and free T3 values were closely correlated with the degree of liver damage, as indicated by elevation of serum bilirubin (r equal -0.547) and reduction of serum albumin (r equal 0.471). The mean serum TSH level was 3.1 muU/ml in the normals and 7.1 muU/ml in the cirrhotic aptients ( less than 0.001). 15% of the hepatic patients had serum TSH values above 10 muU/ml, which, however, did not correlate with any of the four liver function tests studied. Serial blood sampling from two convalescing patients with alcoholic hepatitis showed a gradual normalization of serum TSH and T3 levels as the liver function improved. After oral T4 administration, 0.25 mg/day for 10 days, three of four cirrhotic patients studied failed to raise their serum T3 values. The mean T4 to T3 conversion rate of seven normal subjects was 35.7%. The mean T4 to T3 conversion rate of four cirrhotic patients studied was significantly reduced to 15.6% (P less than 0.001). The mean disposal rates of T4 and T3 of the normal subjects were 114 and 34 mug/day, respectively. The ratio of T4 disposal to T3 disposal was 3.5. In contrast, the mean T4 disposal rate, 82 mug/day, and the mean T3 disposal rate, 10 mug/day, were both reduced in the cirrhotic patients. Their ratio of T4 disposal to T3 disposal was 7.9. These findings suggest that impairment of T4 conversion in patients with advanced hepatic cirrhosis may lead to reduced T3 production and lowered serum T3 level. Therefore, the liver is one of the major sites of T4 conversion to T3.

Conversion of thyroxine to triiodothyronine by cultured human cells

Human liver and kidney cells convert 6 to 10 percent of added thyroxine to triiodothyronine in vitro at 37 degrees C. This extent of conversion is ten times greater than that in control studies with killed cells. Conversion is evident within 10 minutes and appears to be maximal within 1 hour. Greater net triiodothyronine formation results if greater amounts of exogenous thyroxine are added to the system, with no plateau evident even at very high thyroxine concentrations. The addition of high concentrations of nonradioactive triiodothyronine resulted in no evident inhibition of the conversion.

Studies of isolated fetal mouse hearts in organ culture. Evidence for a direct effect of triiodothyronine in enhancing cardiac responsiveness to norepinephrine

Definitive confirmation or denial of the hypothesis that thyrotoxic hearts are supersensitive to catecholamines has been difficult to obtain, largely because secondary alterations in neural and humoral factors that occur after thyroid administration in vivo may obscure the primary changes induced by the hormone on the myocardium itself. To study the direct action of thyroid hormone apart from secondary factors, thyrotoxicosis should be induced in isolated hearts in vitro, but the slow onset of thyroid action plus the rapid deterioration of conventional in vitro preparations have precluded such experiments.Recen'ly a method was developed for maintaining intact, spontaneously-beating hearts from late-fetal mice in organ culture for several weeks. When 5 x 10(-7) to 5 x 10(-6)m l-triiodothyronine is added to the culture medium arrhythmias and or tachycardia gradually appear, just as in thyrotoxicosis in vivo. Accordingly, these "thyrotoxic" hearts were used in the present experiments to test for altered responsiveness to norepinephrine. Dose-response curves to norepinephrine were identical for hearts maintained for 3 hr in triiodothyro-nine-treated or control medium. After 2 days, however, the curve was shifted to the left in triiodothyronine-treated hearts. Thus, 10(-8)m norepinephrine increased the atrial rate by 20+/-6.1 (sem) beats/min in hearts exposed to 5 x 10(-6)m triiodothyronine and by 1+/-3.0 in control hearts (P < 0.02); 10(-7)m norepinephrine raised the rate by 79+/-22.3 in treated hearts vs. 17+/-9.8 in controls (P < 0.05). At maximal doses (10(-6)m norepinephrine), increases were identical. In addition, norepinephrine (10(-8)-10(-6)m) induced arrhythmias in 56% of treated hearts vs. 14% of controls (P < 0.01). Thus, in a precisely controlled environment free of differences in neural and humoral factors, triiodothyronine can act directly on the fetal mouse heart to enhance sensitivity to norepinephrine.