Pamela Bacharach

Low Serum Thyrotropin Concentrations as a Risk Factor for Atrial Fibrillation in Older Persons

BACKGROUND Low serum thyrotropin concentrations are a sensitive indicator of hyperthyroidism but can also occur in persons who have no clinical manifestations of the disorder. We studied whether low serum thyrotropin concentrations in clinically euthyroid older persons are a risk factor for subsequent atrial fibrillation. METHODS We studied 2007 persons (814 men and 1193 women) 60 years of age or older who did not have atrial fibrillation in order to determine the frequency of this arrhythmia during a 10-year follow-up period. The subjects were classified according to their serum thyrotropin concentrations: those with low values (< or = 0.1 mU per liter; 61 subjects); those with slightly low values (> 0.1 to 0.4 mU per liter; 187 subjects); those with normal values (> 0.4 to 5.0 mU per liter; 1576 subjects); and those with high values (> 5.0 mU per liter; 183 subjects). RESULTS During the 10-year follow-up period, atrial fibrillation occurred in 13 persons with low initial values for serum thyrotropin, 23 with slightly low values, 133 with normal values, and 23 with high values. The cumulative incidence of atrial fibrillation at 10 years was 28 percent among the subjects with low serum thyrotropin values (< or = 0.1 mU per liter), as compared with 11 percent among those with normal values; the age-adjusted incidence of atrial fibrillation was 28 per 1000 person-years among those with low values and 10 per 1000 person-years among those with normal values (P = 0.005). After adjustment for other known risk factors, the relative risk of atrial fibrillation in elderly subjects with low serum thyrotropin concentrations, as compared with those with normal concentrations, was 3.1 (95 percent confidence interval, 1.7 to 5.5; P < 0.001). The 10-year incidence of atrial fibrillation in the groups with slightly low and high serum thyrotropin values was not significantly different from that in the group with normal values. CONCLUSIONS Among people 60 years of age or older, a low serum thyrotropin concentration is associated with a threefold higher risk that atrial fibrillation will develop in the subsequent decade.

Induced metamorphosis in Ambystoma mexicanum: Lack of effect of triiodothyronine on tissue or blood levels of thyrotropin-releasing hormone (TRH)

Abstract In order to test the hypothesis that thyroid hormone affects thyrotropin-releasing hormone (TRH) during metamorphosis, the latter was induced in the facultative neotenic amphibian, Ambystoma mexicanum , by injection of triiodothyronine (T 3 ). Plasma levels of T 3 were readily detectable up to 2 weeks after injection. Immunoreactive TRH was found in large quantities in the hypothalamus and brain, in much smaller amounts in the pituitary, pineal, and skin, and was only occasionally detectable in blood. No changes in tissue or blood levels of TRH were found during metamorphosis after treatment with T 3 , suggesting that any hypothalamic action of thyroid hormone during metamorphosis does not involve TRH in this species.

Thyrotropin-releasing hormone and thyrotropin in the control of thyroid function in the turtle, Chrysemys picta ☆

Abstract In adult male Chrysemys picta , while ovine thyrotropin (TSH) causes a clear rise in plasma thyroxine (T 4 ), thyrotropin-releasing hormone (TRH), given in large doses for several days (50 μg/day for 8 days), does not. Neither affected plasma corticosterone. These findings suggest that the repitilian pituitary-thyroid axis is similar to that of other ectotherms in its lack of response to TRH; one may speculate that endothermy and thyroid stimulation by TRH are physiologically linked during the course of evolution.

Conversion of thyroxine to triiodothyronine in an adult amphibian, Bufo marinus

Adult toads (Bufo marinus) were injected with T4 (50 μg/day for 5 days) to assess possible conversion of T4 to T3. Plasma T3 was undetectable (<12.5 ng/dl) before injection of T4; the mean corrected plasma level of T3 after injection was 259 ± 83 (SD) ng/dl, indicating in vivo conversion of T4 to T3 in an adult amphibian. Although as yet of uncertain physiologic significance, this conversion may be important in the action of thyroid hormone in adult amphibia.