Letter: Triiodothyronine/reverse-triiodothyronine balance and thyroxine metabolism

Do thyroid function tests independently predict survival in the critically ill?

We studied the ability of thyroid function tests to predict hospital survival in 116 critically ill patients and compared the results with independent predictions of survival made by critical care physicians. Eleven patients (9.5%) had clinically unsuspected hypothyroidism and were less likely to survive (p = 0.03). In patients critically ill with nonthyroidal disease, low T3, low FT3I, low T4, low FT4I, high TSH, and high T3U levels each showed significant correlation with nonsurvival (all p less than 0.02). Of these, however, only low T3 (p less than 0.001) and high TSH (p = 0.016) showed significant independent prediction of nonsurvival, and only low T3 (p = 0.011) added any significant independent prediction of nonsurvival beyond that made clinically by the group of critical care physicians.

Thyroid function in dietary amenorrhoea

Abnormalities of thyroid function are well known to occur in anorexia nervosa. A commoner problem is the patient with secondary amenorrhoea due to weight loss, who does not have true anorexia nervosa. Eight such patients were studied and compared to a normal control group. Total and free thyroxine, total and free triiodothyronine were found to be significantly lower in the amenorrhoea patients. No difference in basal thyroid stimulating hormone or reverse triiodothyronine levels were found. In three of four patients who had thyrotrophin releasing hormone tests performed a prolonged response was found. These results indicate that abnormal thyroid function tests may be found in this milder form of the disease. In addition, the normal reverse triiodothyronine levels suggest the possibility of two mechanisms existing, whereby T3 levels may be lowered in nutritional disorders.

No inhibition by Li+ of thyroxine monodeiodination to 3,5,3'-triiodothyronine and 3,3',5'-triiodothyronine (reverse triiodothyronine)

The possibility that lithium affects the conversion of thyroxine to 3,5,3"-triiodothyronine and 3,3',5'-triiodothyronine (reverse triiodothyronine) was studied by measurement of the serum concentractions of these parameters in five patients during the first week of lithium therapy. In three patients there was a decrease in serum thyroxine concentration and a slightly less pronounced decrease in that of serum 3,5,3'-triiodothyronine. In two patients, who also received L-tryptophan or flupentixol, no change was noted in the concentrations of these compounds. There was no increase in serum 3,3',5'-triiodothyronine concentration in any of the patients. No systematic change was found in the serum concentrations of thyrotropin or unsaturated thyroid-hormone binding proteins. The results obtained do not support the contention that lithium should inhibit the monodeiodenation of thyroxine to its active and inactive metabolites.

The thyroidal production of reverse triiodothyronine in autonomous adenoma

The ratios of reverse triiodothyronine (rT3) to triiodothyronine (T3) concentrations were similar (0.89 +/- 0.09) in twenty-three autonomous thyroid nodules (fourteen decompensated, nine compensated) and paranodular tissues (0.82 +/- 0.08), whereas the serum ratio was significantly lower (0.098 +/- 0.014). This is compatible with the hypothesis that thyroidal rT3 and T3 production from mono- and diiodotyrosine may be a random process in contrast to non-random extrathyroidal rT3 and T3 formation.

3,3',5'-Triiodothyronine (Reverse T3) in amniotic fluid and cord serum

3,3',5'-triiodothyronine (reverse T3, rT3) was measured in 122 samples of amniotic fluid obtained between the 29th--40th weeks of gestation, and in the blood of 37 newborn and their mothers. The mean rT3 concentration in amniotic fluid was 0.769 +/- 0.47 ng/ml, with a slight decrease from the 29th--40th weeks of gestation which was not statistically significant. Because of the great scatter of rT3 concentrations in amniotic fluid, its estimation does not seem to be useful in prenatal diagnosis of hypothyroidism. The mean rT3 concentration in cord blood (2.62 ng/ml; range 1.4--4.9 ng/ml) was greatly elevated in comparison to the mean maternal level at delivery (0.34 ng/ml; range 0.08--0.69 ng/ml). The possible use of rT3 measurements in cord blood as a screening test for congenital hypothyroidism is discussed.

The radioimmunoassay of 3,3',5' - triiodothyronine (reverse T3) in unextracted human serum

A specific, sensitive and simple double antibody radioimmunoassay for total serum 3,3',5'-triiodothyronine (reverse T3, rT3) in small volumes of unextracted human serum is described. High titre antisera were raised in rabbits using dl=rT3 or l-rT3 conjugated to bovine serum albumin. The selected antisera cross reacted less than 0-003% with triiodothyronine (T3) and 0-14% with thyroxine (T4). A stable high activity rT3 tracer was prepared by iodination of 3,3"-diiodo-l-thyronine by the chloramine-T method, and purified by column chromatography on Sephadex LH-20. Binding of rT3 to endogenous serum proteins was blocked by including 8-anilino-1-naphthalene sulphonic acid (ANS) in the assay. Mean rT3 levels in healthy euthyroid adults are 0-27 nmo1/1 (range 0-21-1-18); in cord sera 3-67 nmo1/1 (range 2-30-7-45) and in amniotic fluid taken during the second trimester 4-70 nmo1/1 (range 2-22-8-00). In approximately half the hypothyroid subjects, rTO levels were undetectable ( less than0-05 nmo1/1) and in the remaining subjects the mean rT3 level was 0-10 nmo1/1 (range 0-05-0-25).