Abstract
Objective: Glucagon has been proposed to contribute to the increased glucose production found
in hyperthyroidism. However, fasting plasma glucagon levels are not increased in hyperthyroidism
suggesting that the activity of the α-cell is normal. Nevertheless, an increase in
the clearance rate of glucagon may mask increased glucagon secretion. This study was
designed to examine the effects of hyperthyroidism on the kinetics of glucagon.
Design and methods: A primed-continuous infusion of glucagon was administered to 9 euthyroid and 9 hyperthyroid
subjects at 3 sequential rates (1 200, 3 000 and 6 000 pg/kg/min, each given for 2 h).
Arterialized blood was drawn at 15–30 min intervals for determination of glucagon.
Results: Fasting plasma glucagon levels were comparable in euthyroids (195±8 pg/ml) and hyperthyroids
(231±16 pg/ml). During infusions (1 200, 3 000 and 6 000 pg/kg/min), plasma glucagon
increased to 387±19, 624±44 and 977±51 pg/ml in euthyroids and to 348±23, 597±42 and
938±56 pg/ml in hyperthyroids respectively. At these infusion rates, metabolic clearance
of glucagon (ml/kg/min) was 6.6±0.5, 7.4±0.6 and 7.9±0.5 in euthyroids and 12.6±2,
8.9±1 and 8.8±0.6 in hyperthyroids, respectively. Metabolic clearance of glucagon
differed between hyperthyroids and euthyroids at 1 200 pg/kg/min infusion rate (p=0.001).
The basal delivery rate of glucagon (ng/kg/min) was 1.3±0.1 in euthyroids and 2.9±0.6
in hyperthyroids (p=0.0005).
Conclusions: In hyperthyroidism, the secretion and metabolic clearance rates of glucagon are increased.
These effects may explain the changes in plasma glucagon levels observed in hyperthyroidism
and support the important role of glucagon in increasing endogenous glucose production
in this condition.
Key words
endogenous glucose production - insulin resistance - thyroid hormones
References
Correspondence
gdimi@ath.forthnet.gr gdimitr@med.uoa.gr
