Leptin signals the status of energy reserves to the brain. Leptin stimulates biosynthesis of TRH in vitro and influences the activity of the hypothalamic-pituitary-thyroid axis in vivo in rodents. Because blood levels of both leptin and TSH display diurnal variation with a distinct nocturnal rise, we sought to determine whether a relationship exists between fluctuations in circulating leptin and TSH. We measured serum leptin and TSH levels every 7 min for 24 h in five healthy men and found that both leptin and TSH levels are highly organized and pulsatile. A similar pattern of leptin and TSH rhythms was observed, with TSH and leptin levels reaching a nadir in late morning and a peak in the early morning hours. Importantly, cosinor analysis on the absolute leptin and TSH levels revealed a statistically significant fit for a 24-h period and the two hormones showed similar probabilities of rhythm and superimposable peak values. Furthermore, this study shows a strong positive Pearson correlation between the 24-h patterns of variability of leptin and TSH in healthy subjects. Finally, the ultradian fluctuations in leptin levels showed pattern synchrony with those of TSH as determined by cross-correlation analysis, by cross-approximate enthropy and Bayessian analysis applied independently. To further explore whether these associations could reflect an underlying regulation of TSH secretion by leptin, we also studied frequently sampled leptin and TSH levels in four brothers, members of a family with leptin deficiency (one normal homozygote, two heterozygotes, and one leptin-deficient homozygote). Leptin levels of the homozygous leptin-deficient subject are detectable but bioinactive, and the rhythm of his TSH is disorganized. 24-h pattern of leptin and TSH variability in the heterozygous subjects, although significantly correlated, showed a weaker correlation compared with the strong correlation in the normal subjects. These data are consistent with the possibility that leptin may regulate TSH pulsatility and circadian rhythmicity, but interventional studies are needed to definitively prove whether leptin regulates the minute-to-minute oscillations and ultradian rhythm of TSH levels.