Thyroid function and thyroid hormone metabolism in elderly people. Low T3-syndrome in old age?

T4-, T3- and reverse-T3 concentrations were measured in the sera of 365 subjects beyond the age of 65 in order to evaluate if the decrease of serum T3 frequently observed in old age can be attributed to old age per se or to concomitant nonthyroidal disease. The results obtained from a carefully selected healthy group of elderly people show that 1) total and free T3 levels are lower in senescence but well within the range for euthyroidism in younger healty controls;2) the decrease of serum T3 is more pronounced and occurs earlier in healthy old males than in females, so that for subjects over the age of 75, the upper limit for euthyroidism has to be adjusted by 10% in women and by 20% in men; and 3) there is no low T3 syndrome characterized by decreased serum T3 and increased serum reverse T3, solely due to old age. Turnover kinetics have shown the daily production of T4 and T3 in old age to decrease by 20 micrograms and 10 micrograms, respectively, and an increased T3 metabolic clearance not to account for the reduction of serum T3 concentrations. Combined stimulation tests with TSH and TRH showed that the functional reserve of the thyroid gland to produce T3 is maintained in old age. The first step in the sequence of events may be seen in an impairment of TSH secretion leading to an adaptation of the amount of thyroid hormones to a reduced mass of metabolically active body tissue in old age.

[False radioimmuno-assay of thyroxine and triiodothyronine in the presence of hormone-binding autoantibodies in serum (author's transl)]. / Radioimmunologische Fehlbestimmung von Thyroxin und Trijodthyronin bei Anwesenheit hormonbindender Autoantikörper im Serum

Radioimmunoassay of thyroxine and triiodothyronine in a 14-year-old girl with primary hypothyroidism and nodular goitre as a result of Hashimoto's thyroiditis gave falsely low values due to the presence of hormone-binding antibodies. Such antibodies occur in Hashimoto's thyroiditis and thyroid carcinoma. Their presence requires special methods for determining these hormones.

[Measurement of thyroxine binding globulin by competitive ligand binding assay (CLBA) (author's transl)]. / Bestimmung des Thyroxin-bindenden Globulins (TBG) mit Hilfe des Competitive Ligand-Binding Assay (CLBA): Methodik und Ergebnisse

The competitive ligand binding assay (CLBA) first described by Chopra et al. ((1972) J. Clin. Endocrinol. Metab. 35, 565-573) is a convenient routine method for the accurate measurement of thyroxine binding globulin in large numbers of serum samples. The assay is based on the partition of a constant quantity of radiolabelled T3 between a fixed quantity of rabbit T3 antibodies and the thyroxine binding globulin of the serum, after prior removal of T3 and T4 from the serum with an anion exchange resin (Amberlite IRA 400). In euthyroid subjects serum thyroxine binding globulin was 25.5 +/- 5.0 mg/1, in hyperthyroid patients thyroxine binding globulin was significantly decreased to 13.0 +/- 4.0 mg/1 and was significantly increased in hypothyroid patients to 36.8 +/- 6.2 mg/1 as well as in pregnant women to 41.3 +/- 6.2 mg/1. No difference was found between normal subjects and young women taking contraceptive pills with low oestrogen content. There were significant negative correlations between the thyroxine binding globulin in serum on the one hand side and the free T4-and free T3-fraction on the other. The low thyroxine binding globulin estimates in hyperthyroid patients increased gradually to normal during treatment with thyroid blocking drugs, the elevated thyroxine binding globulin in hypothyroid patients decreased to normal during treatment with thyroid hormones. The competitive ligand binding assay used here seems to be convenient as a routine method for the precise and reproducible measurement of thyroxine binding globulin in serum.

[Excessive peripheral conversion of thyroxine (T4) to triiodothyronine (T3) in the pathogenesis of T3-hyperthyroidism (author's transl)]. / Exzessive periphere Konversion von Thyroxin (T4) zu Trijodthyronin (T3) in der Pathogenese der T3-Hyperthyreose

In a 41-year-old woman and a 3-year-old girl, both of them with T3-thyrotoxicosis, serum levels of total and free T4 and T3 were measured serially during anti-thyroid drug treatment. Attempts to substitute thyroxine during the antithyroid treatment had to be interrupted because the patients became hyperthyroid again with excessive increases in total and free serum T3, even when concentrations of total and free T4 were brought to subnormal levels. The increased conversion of administered thyroxine ceased later on and higher amounts of oral T4 were tolerated after one year of treatment. In both patients there was an extremely low serum T4/T3 ratio, differing in this respect significantly from six other patients with T3-thyrotoxicosis and 41 patients with "conventional" T3/T4-hyperthyroidism. It is concluded that, in patients with T3-thyrotoxicosis due to excessive peripheral T4 to T3 conversion, substitution during antithyroid drug treatment should be either with very low doses of thyroxine or with triiodothyronine in divided daily doses. In such cases the level of serum T3 represents the most reliable biochemical measurement for the control of treatment, serum T4 levels being irrelevant.