Urinary tri-iodothyronine excretion as index of thyroid function

Activity of the hypothalamic-pituitary-thyroid axis during exposure to cold

It seems clear from the studies reviewed here that there is adequate evidence to support the concept of a biphasic response of the thyroid gland to cold as first postulated by Moll et al. (1972). The initial response to acute exposure to cold begins at the level of the hypothalamus as a result of either neural stimuli from skin and other areas and/or blood of somewhat lower than normal temperature reaching the hypothalamus (Andersson et al., 1963). As a result, the secretion of norepinephrine and/or dopamine may increase, and serotonin and/or somatostatin may decrease. The net result of these is an increase in the release of TRH from the hypothalamus. This, in turn, stimulates the cascade for the release of TSH from the anterior pituitary gland and thyroid hormone from the thyroid gland. Moll et al. (1972) postulated the lack of a feedback limb in this acute phase, and, indeed, this may be the case. It is possible, however, that certain hormones, such as somatostatin, norepinephrine, T3, and T4 could act in the capacity of feedback inhibitors. Additional experiments will be required to assess this possibility. The transitional link between the acute (less than 1 day) and chronic (greater than 1 day) phases of the response of the thyroid gland to cold could be T4 itself. An increase in the concentration of T4 in plasma has been reported to increase peripheral deiodination of T4 to T3 by kidneys and liver of rats. There are no studies at present to indicate that hepatic conjugation can be increased by elevation of plasma levels of T4 and T3. If it can, these responses would provide adequate reasons as to why peripheral metabolism of thyroid hormones increases during chronic exposure to cold. The time-course for these changes to occur needs to be studied in greater detail to establish the sequence of events following acute exposure to cold. The latter may also increase urinary excretion of T4 and T3 in man, but not the rat. This suggests that another aspect of exposure to cold needing additional study is measurement of the binding affinities of T4 and T3 for their transport proteins during exposure to cold as compared to affinities prior to exposure to cold. If binding affinities are reduced, the amount of free hormones would increase and, consequently the likelihood of being excreted into urine and conjugated by the liver would also increase.(ABSTRACT TRUNCATED AT 400 WORDS)

The involvement of the pentose shunt in thyroid metabolism after stimulation with TSH or with immunoglobulins from patients with thyroid disease. II. The reoxidation of NADPH and stimulation of hormone synthesis

Reducing equivalents derived from the tissue re-oxidation of NADPH (NADPH-diaphorase) have been implicated in the peroxidation that is involved in the organification of iodine in the production of thyroid hormones. Immunoglobulin (Ig) fractions from patients with thyroid diseases and from normal controls, in a standard dose of 125 micrograms/ml and 0.3 microunits/ml thyrotrophin (TSH) were incubated with segments of guinea-pig thyroid gland maintained in vitro. A quantitative cytochemical study was made on how these fractions influenced the enzyme activity. A good correlation was found between the ability of such Ig fractions to stimulate the NADPH-diaphorase activity and (1) the degree of hyperthyroidism in the patients and (2) the amount of T3 secreted by the thyroid segments in vitro.

The measurement of thyroxine in urine using a competitive protein binding technique

The competitive protein vinding assay for the measurement of thyroxine in blood serum was modified for the measurement of thyroxine in urine. Samples of urine of 1 to 5 ml volume containing 0 to 5 ng/ml could be assayed with 100% recovery, and above this range, up to 10 ng, recoveries were higher due to non parallelism with the standard curves. Tests carried out using porcine serum albumin indicate that results obtained by the method are not likely to be affected by proteinuria. The cross reaction with triiodothyronine was 25%. Analysis of urine samples stored at 25 degrees C gave higher values than those stored at +4 degrees C or --20 degrees C over similar periods. These increases at 25 degrees C were of the same magnitude as those obtained by acid hydrolysis of urinary thyroxine conjugates.

Unconjugated thyroxine and triiodothyronine in urine: influence of age, sex, drugs and thyroid function

The 24 h urinary excretion (dU) of T4 and T3, determined by radioimmunoassays using extraction and separation on Sephadex columns, is higher in men than in women (mean dU-T4 and dU-T3 +/- 2 SD: 2.6 +/- 0.9 vs. 2.2 +/- 1.0 and 2.5 +/- 1.0 vs. 2.0 +/- 0.8 nmol, respectively). The excretion of T3 decreases with age, but expressed relatively to dU-creatinine there is no effect of age and sex. Children have increased T4 and T3 to creatinine excretion ratios and neonates excrete predominantly T4. Oestrogens, acute salicylate loading and diurnal variation yielded results, which were at variance with the hypothesis that dU-T4 and dU-T3 mirror corresponding free hormone concentrations in serum, and there was only a weal positive correlation to total serum hormones in thyrotoxicosis as well. General clinical use of urine T4 and T3 is obviated by poor diagnostic discrimination and inherent analytical and interpretative disadvantages.

The relationship between circadian variations in circulating thyrotrophin, thyroid hormones and prolactin

Half-hourly blood samples were taken from six clinically euthyroid men over a continuous period of 24 h. Their concentrations of total thyroxine (T4), total triiodothyronine (T3), thyrotrophin (TSH) and prolactin (PRL) were assessed together with the degree of unsaturation of thyroid hormone binding proteins as determined by the thyroid hormone uptake test (THUT). Both T3 and T4 were also measured in urine samples collected serially during the same 24 h period. Significant circadian changes in serum TSH, THUT, serum and urine T4 and serum PRL were demonstrated in all subjects. TSH showed a reciprocal pattern to serum T4, with higher levels during the evening and at night than the daytime. This TSH pattern did not coincide with PRL secretion. Further studies on the same subjects did not show any significant effect of posture, corticosteroid or T4 administration upon circadian changes in TSH. There appeared to be no consistent circadian changes in serum or urinary T3. It seems likely that the TSH circadian rhythm is centrally determined and that free T3 levels are maintained more or less constant by variation in peripheral conversion from T4.

Radioimmunoassay of thyroxine and triiodothyronine in urine using extraction and separation of Sephadex columns

Radioimmunological methods for the determination of thyroxine (T4) and triiodothyronine (T3) in urine have been developed. The methods are based on trapping of T4 and T3 from unextracted urine, followed by separation between free and antibody bound hormone on the same Sephadex column. The T4 method has been compared to a competitive protein binding (CPB) assay using ethyl acetate extraction. The methods are evaluated in sixty-seven euthyroid controls, twenty-four hyperthyroid and seven hypothyroid patients. In the T4 RIA detection limit was 3.9 pg, intra-assay coefficient of variation (cv) was 5.2% and inter-assay cv was 6.9%. In the T3 RIA detection limit was 7.2 ng, intra-assay cv 3.9% and inter-assay cv 10.8%. Recovery of added amounts of hormones and serial dilutions gave satisfactory results. The CPB assay was found unreliable with unspecific and false high values. In euthyroid controls 24 h urinary T4 excretion as measured by RIA was 1.8 +/- 0.5 nmol, and urinary T3 excretion was 0.7 +/- 0.3 nmol. T4 and T3 excretion was greatly elevated in hyperthyroid patients and decreased in hypothyroidism.

Thyroid function in changing weather in a subtropical region

Serum and 24-hr urine samples were collected on 2 consecutive days during the first week of each month for 1 yr from eight healthy euthyroid men aged 25-37 yr. The means of minimum and maximum environmental temperature for the 30 days period preceding the sample collection represented the temperature for that month. Total serum thyroxine (T4), triiodothyronine (T3), thyrotropin (TSH), and urinary T3 and T4 were measured by specific radioimmunoassays and serum thyroxine-binding globulin (TBG) by the radioligand binding assay. The serum TSH and urinary T3 and T4 responses to 100 mug intravenous TRH were studied in five subjects during summer and again during winter. The serum concentration of these hormones and TBG did not reveal significant variations throughout the year. However, the mean urinary excretion of both T3 and T4 during coldest months (January and February), at 0.97 and 1.95 mug/24 hr, respectively were significantly higher than the corresponding values (T3, 0.48; T4, 1.18 mug/24 hr) during the hottest months (May-July). The TSH, and urinary T3 and T4 responses to identical doses of TRH during summer and winter did not differ significantly. Since urinary T3 and T4 indirectly reflect the prevailing unbound serum levels of these hormones, it is likely that the greater availability of free and biologically active thyroid hormones could help the body to adapt to cold by increasing nonshivering thermogenesis.

Assessment of thyroid function during pregnancy

Serum levels of thyroid stimulating hormone and of the two thyroid hormones were compared in pregnant women and in non-pregnant control subjects. Circulating levels of thyroid stimulating hormone did not alter significantly throughout pregnancy. Total thyroxine and triiodothyronine concentrations increased markedly during the first and second trimesters, associated with a progressive increase in the thyroid binding capacity of serum proteins. Urinary triiodothyronine and thyroxine excretion during the third trimester of pregnancy was comparable to that found in non-pregnant euthyroid females and, it is concluded, provides the simplest method of assessing thyroid function during pregnancy.

The assay of urinary thyroid hormones for assessing thyroid function

A competitive protein binding assay for thyroxine (T-4) and a radioimmunoassay for tri-iodothyronine (T-3) were applied to determine the urinary concentrations of these hormones. These measurements were shown to offer excellent diagnostic discrimination between euthyroid, hypothyroid, and hyperthyroid subjects. Urinary T-3 and T-4 levels were not affected by changes in the concentration of thyroxine binding globulin (TBG) or by changes in binding capacity; and therefore, remained normal during pregnancy, oestrogen, and phenytoin therapy. There was no significant circadian variation in urine T-3 excretion. Thus T-3 assays of overnight urine samples could be used in conjunction with dynamic test procedures, such as thyrotrophin (TSH) stimulation test, to enable the rapid assessment of the integrity of the hypothalamic-pituitary-thyroid axis.