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In order to clarify the role of the pituitary conversion of L-thyroxine (T4) to 3, 5, 3'-L-triiodothyronine (T3) in regulating thyrotropin (TSH) secretion, the effect of altered thyroid states and fasting on intrapituitary T3 generation was investigated by a paperchromatographic procedure using the anterior pituitary homogenates. Hepatic T3 generation was also studied for comparison. The rate of pituitary and hepatic T3 generation in normal rats averaged 25.2 +/- 12.4 (mean +/- SE) fmoles T3/mg protein/min and 33.8 +/- 12.7 fmoles T3/mg protein/min, respectively. T4 treatment raised the hepatic T3 generation from T4 (46.7 +/- 3.1 fmoles T3/mg protein/min) and lowered the intrapituitary T3 generation (4.5 +/- 0.2 fmoles T3/mg protein/min). On the contrary, thyroidectomy slowed the hepatic T4 5'-deiodination (11.0 +/- 2.8 fmoles T3/mg protein/min), and accelerated the pituitary T4 5'-deiodination (64.3 +/- 1.4 fmoles T3/mg protein/min). In 48 h fasted rats, serum T4, T3 and TSH concentrations were all lower than those in fed rats, and both pituitary and hepatic T3 generations were also suppressed. Thus, altered thyroid states cause an opposite effect on pituitary and liver 5'-monodeiodination, whereas fasting causes similar changes. The findings suggest the existence of an autoregulatory mechanism for thyroid hormone activation within the target tissues.

Modulation of T4 5'-monodeiodination in rat anterior pituitary and liver homogenates by thyroid states and fasting.