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ANANDA NIMALASURIYA, CAROLE A. SPENCER, STEVEN C. LIN, JACQUELINE K. TSE, JOHN T. NICOLOFF, Studies on the Dirunal Pattern of Serum 3,5,3′-Triiodothyronine, The Journal of Clinical Endocrinology & Metabolism, Volume 62, Issue 1, 1 January 1986, Pages 153–158, https://doi.org/10.1210/jcem-62-1-153
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Abstract
Seventeen subjects, including 12 mildly obese women, were investigated to determine the presence of a diurnal variation in serum T3 levels. A subset of 8 subjects was studied after fasting for 6 days, while another subset of 5 subjects was studied 2 days after receiving 3 mg T4, orally, to suppress TSH secretion. To negate the influence of hemoconcentration produced by ambulation, serum T3 (toT4 ratios (nanograms per μg) rather than total T3 values were used for analysis. A synchronous (diurnal rhythm for mean serum T3 to T4 ratios and TSH values was found, with mean nighttime increases of 7.8% and 49.5%, respectively. The timing of the T3 to T4 and TSH nocturnal peaks for individual subjects, however, were not correlated,(suggesting that T3 (to T4 ratios changes were not caused by TSH stimulation of thyroid T3 release. During fasting, the diurnal rhythm of serum T3 to T4 was obliterated within 24 h of beginning the fast, although TSH rhythmicity persisted. After the 3-mg oral (T)4 dose, serum TSH became undetectable, while the (diurnal serum T3 (to T4 ratio changes persisted. In contrast, there was no detectable diurnal rhythmicity of serum T3 to T4 ratios during either the control or fasting period. We conclude that the diurnal rhythm of serum T3 is not TSH dependent, but, rather, i s influenced by some as yet unidentified dietary signal, which alters the efficiencyof the peripheral tissue T4 to T3 conversion. (J ClinEndocrinol Metab62: 153, 1986)