Plasma Triiodothyronine Concentrations Research Articles

Thyroid function and blood pressure in two new strains of spontaneously hypertensive and normotensive rats.

1. The influence of thryoid function on the development of hypertension was studied in strains of spontaneously hypertensive (SH) and normotensive rats. 2. Surgical thyroidectomy decreased systolic blood pressure more markedly in SH rats than in normotensive rats. The effects of oral administration of 5 and 100 micrograms of thyroxine 24 h-1 100 g-1 were studied in the thyroidectomized animals. In the two strains the blood pressure returned to control levels only after administration of the larger dose. 3. The evolution of body weight, total plasma tri-iodothyronine (T3) and tetraiodothyronine (T4) concentrations were followed as a function of age in SH rats and normotensive rats from 5 to 21 weeks. At each age, SH rats showed significantly larger body weight and decreased T4 concentrations. Plasma T3 in SH rats was lower than in normotensive rats until 15 weeks of age, after which the difference was not significant. At 11 weeks, plasms free T3 and T4 concentrations were slightly lower in SH rats than in normotensive rats. 4. The more marked hypotensive effects of surgical thyroidectomy in SH rats cannot be related to increased thyroid function.

Recovery of peripheral nerve function after axotomy: Effect of triiodothyronine

The rate of recovery of motor function after crushing the sciatic nerve was measured in rats treated with daily intraperitoneal injections of 3,3′,5-triiodo- l-thyronine, 25 μg/kg. The nerve was crushed at one of four levels, and recovery of motor function was indicated by the return of toe spreading elicited as part of the vestibular placing response. The rate of recovery was calculated from the linear regression of the time to recovery on the distance from the lesion to the entry of the peroneal nerve into the anterior tibial muscle. The recovery rate of the triiodothyronine-treated group was 4.08 ± 0.22 ( se) mm/day, an increase of 22% from the rate of 3.35 ± 0.14 mm/day ( P = 0.006) in the vehicle-injected control rats. The latent periods for the two groups were not significantly different. The outgrowth of the most rapidly growing sensory axons was measured using the “pinch test” on the 7th or 9th day after sciatic crush in animals treated with triiodothyronine in dosages from 1 to 400 μg/kg/day and compared with control animals. No significant difference in distance regenerated or rate of regeneration was found. Control nerves regenerated at a rate of 4.5 ± 0.15 mm/day after a delay of 2.0 ± 0.2 days. A significant degree of hyperthyroidism was documented by measurements of plasma triiodothyronine concentrations taken 4 and 18 h after the injection of the hormone, 25 μg/kg. At the same dosage animals gained weight normally and showed no signs of thyrotoxicity. These results are interpreted as showing that the accelerated recovery of motor function in the hyperthyroid rat is mediated via an effect on the maturation of the regenerated axon rather than on axonal outgrowth. A triiodothyronine-related increase in the rate of myelination would be consistent with the observed results.

The Effect of Basal Hypothalamic Isolation on Pituitary-Thyroid Activity and the Response to Propylthiouracil

Basal hypothalamic deafferentation extending from the posterior border of the optic chiasm to the mid-mammillary bodies resulted in depression of plasma TSH, thyroxine (T4), and triiodothyronine (T3) concentration to 50% of normal controls within 7 days. Administration of 0.15% propylthiouracil (PTU) in the diet form postoperative day 26 caused a pronounced drop in the plasma T3 level and a rise in plasma TSH level within two days in the control animals, but had little effect during this interval in the deafferented animals. After 12 days of PTU, plasma T3 and T4 concentrations had dropped to undetectable concentrations in the control animals but both were still detectable in the deafferented animals. After 25 days of PTU, plasms T4 and T3 levels were undetectable and plasma TSH levels were significantly elevated above normal in all animals. Thyroid hypertrophy at that time was as great in the deafferented as in the control rats, although plasma TSH concentration was 50% lower in the former. Administration of 0.1 mug/100 g BW TRH iv on postoperative day 37, when plasma T4 and T3 were undetectable in the controls but still present in the deafferented animals, produced an equally high concentration of plasma TSH in all animals. We interpret these data to support the concepts that: 1) a major source of neural drive of that TRH which stimulates the secretion of TSH by the adenohypophysis lies outside the medial basal hypothalamus, 2) a decrease in TRH reaching the adenohypophysis causes a lower setting of the "thyrostat" sensitive to the concentration of circulating thyroid hormone, and 3) increased TSH secretion and resultant goitrogenesis is delayed in animals with impaired TRH secretion because of the slower rate of secretion of thyroid hormone than in intact controls and the longer time thus required to markedly reduce the concentration of circulating thyroid hormone.

An increase of plasma triiodothyronine concentration in man in a cold environment.

In an attempt to study an effect of cold on endocrine function in man, plasma thyroxine (T4), triiodothyronine (T3), TSH, and cortisol concentrations were studied in 24 healthy men who worked in a cold environment (4 to 6C) for 3 h for the culture of mushrooms and 56 healthy men who lived in cold environment in winter. For comparison, similar measurements were made on 47 university employees who lived in rooms with air-conditioning. In 24 adult men, acute exposure to cold for 3 h failed to affect plasma concentrations of T4, T3, and TSH. In 56 adult men, no significant difference was found in the concentrations of T4, TSH, and cortisol between summer and winter. However, plasma T3 concentration increased significantly in winter, suggesting seasonal variation of T3. In contrast, no such seasonal variation was found in 47 university employees who lived in rooms with air-conditioning.

T4 thyrotoxicosis with normal or low serum T3 concentration.

Eight patients are described with elevated plasma thyroxine (T4) concentrations, but normal or low plasma triiodothyronine (T3) concentrations by radioimmunoassay. All were suffering also from non-thyroidal illness, which in six cases was severe, and in two fatal. The two patients in reasonable general health were acromegalic. It is possible that in sick thyrotoxic patients there is an impairment of peripheral conversion of T4 to T3, similar to that occurring in sick euthyroid patients. However, further proof of thyrotoxicosis must be obtained in patients with high T4 and normal or low T3 levels by studying the response of the pituitary to TRH. Meanwhile, in the sick or elderly, estimation of serum or plasma T3 concentration may not be a good screening test for thyrotoxicosis.

Prostaglandins stimulate thyroid function in pregnant women.

Intravaginal or intravenous administration of prostaglandin F(2alpha) to pregnant women produced significant elevations of plasma triiodothyronine and thyroxine concentrations, but no rise of thyrotropin. These observations are consistent with those of previous in vitro studies indicating that prostaglandins can act directly on the thyroid gland to stimulate thyroid hormone synthesis or release, or both.

A radioimmunoassay study and comparison of seasonal variation in plasma triiodothyronine and thyroxine concentrations in normal healthy persons

The serum triiodothyronine (T3) and thyroxine (T4) concentration of healthy persons remains almost constant for at least ten months. Fresh serum and serum frozen for two years were compared. The mean difference in T3 concentration is negligible, only 3 ng/100 ml. The radioimmunoassay has a Between Assay Coefficient of 6.3% and is therefore reliable. Results can be obta

Plasma triiodothyronine concentration in the newborn calf.

On a mesure le taux de triiodothyronine et de thyroxine dans le plasma du veau nouveau ne pendant ses premiers 11 jours. Le ratio d T3: T4 a eu son maximum au 5e jour.