tonic secretion

Tonic secretion is continuous, ongoing hormone output that provides a steady background level, as opposed to release that comes only in occasional bursts. The word tonic here means sustained and constant — a hormone that is always on, supplying a baseline tone the body relies on, much as a held musical note underlies a melody.

The concept is especially useful for describing control that runs by constant restraint. The hypothalamus exerts tonic inhibition of prolactin through a steady stream of dopamine, so prolactin is continuously held down; relieve that constant brake and prolactin rises. Tonic and phasic are often paired as opposites: tonic is the steady ongoing level, phasic is the episodic surge layered on top.

Tonic and basal secretion overlap and are sometimes used interchangeably, but the emphasis differs slightly. Basal stresses the low, unstimulated baseline, whereas tonic stresses that the output is continuous and ongoing — a constant tone — which is why control by tonic inhibition behaves so differently from control by intermittent stimulation.