Abstract
A study of diurnal patterns of serum hormone levels was made in genetically lean and
obese Zucker rats. Serum glucose and insulin were elevated in the obese through the
24-hour period; immunoreactive insulin being 12 times higher in the obese rat. A similar
pattern of serum hormone levels in lean and obese rats was found for triiodothyronine,
thyroid stimulating hormone and prolactin. At peak levels, prolactin was 2.5 times
higher in lean than obese rats (P<.01). Thyroxine levels tended to be lower in the
obese rats throughout the sampling periods when compared to lean rats. Growth hormone
levels were severely depressed in the obese rat with the largest differences between
lean and obese rats being observed during the light periods of the cycle. These differences
in growth hormone secretion may be responsible for the impaired nitrogen retention
in the obese rat. Serum levels of corticosterone were highest at the beginning of
the dark cycle and decreased drastically thereafter in the lean rats. The obese rat
did not exhibit a distinct pattern of corticosterone secretion and tended to be elevated
throughout the periods tested.
Key words
Obesity - Hormones - Diurnal Pattern - Growth Hormone - Corticosterone - Thyroid
1 Authorized for publication on December 9, 1976 as paper no. 5208 in the journal series
of the Pennsylvania Agriculture Experiment Station. Supported in part by NIH Grant
No. HD 0709-04.
