There Is a Nyctohemeral Rhythm of Type II Iodothyronine 5′-Deiodinase Activity in Rat Anterior Pituitary*

We found a nyctohemeral rhythm of type II iodothyronine 5'-deiodinase (5'-D-II) with a zenith at midnight in rat anterior pituitary, but not in brown adipose tissue. There was no nyctohemeral rhythm of 5'-D-I in anterior pituitary, liver, or kidney. Hypothyroidism abolished the nyctohemeral rhythmicity in anterior pituitary 5'-D-II. The rhythmicity of anterior pituitary 5'-D-II may play a role in setting the nyctohemeral rhythm of TSH secretion by regulating the degree of negative feedback by locally generated T3 in the thyrotroph.