Studies on the diurnal pattern of serum 3,5,3'-triiodothyronine

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 to T4 ratios (nanograms per microgram) 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 T4 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, is influenced by some as yet unidentified dietary signal, which alters the efficiency of the peripheral tissue T4 to T3 conversion.