text: The secretion pattern of thyrotropin (TSH) is shaped by infradian, circadian and ultradian rhythms. Infradian rhythmis are mainly represented by circannual variation mirroring the seasonality of thyroid function. Circadian rhythms lead to a peak secretion (acrophase) around midnight and nadir concentrations around noon and in the early afternoon. A similar pattern is observed for triiodothyronine, however with a phase shift. Pulsatile release contributes to the ultradian rhythm of TSH concentration with about 10 pulses per 24 hours. The amplitude of the circadian and ultradian rhythms is reduced in severe non-thyroidal illness syndrome (TACITUS).
  Contemporary theories assume that autocrine and paracrine (ultrashort) feedback mechanisms controlling TSH secretion within the anterior pituitary gland are a major factor contributing to the evolution of its pulsatility.