The effect of altered thyroid status on pituitary hormone messenger ribonucleic acid concentrations in the rat

To study the effects of altered thyroid status on pretranslational control of pituitary hormones, adult male rats were given propylthiouracil for 6 weeks and underwent the following studies. 1) Rats were injected with T3 at 10 μg/100 g BW daily for 10 days. 2) Rats were given T3 injections at 0, 0.01, 0.1, 1.0, or 10 μg/100 g BW for 10 days. 3) Rats were killed 0, 1, 6, or 24 h after a single injection of T3 at 10 μg/100 g BW or after 5 or 10 days of daily T3 injections. Pituitary mRNA concentrations of TSH β, α-subunit, PRL, GH, POMC, FSH β, and LH β were determined for individual animals. Marked increases in TSH β and a-subunit mRNAs occurred after PTU treatment, and these changes were reversed by 1.0 μg/100 g BW T3 and within 24 h of a single T3 injection of 10 μg/100 g BW. Further increases in the dose or time course of T3 administration led to a relatively greater suppression of TSH β mRNA levels than α-subunit mRNA levels. In contrast, GH and PRL mRNA levels were low in hypothyroid animals, and both rose toward control levels with 0.1 μg/100 g BW T3 and by 24 h after a single T3 dose. Induction of hyperthyroidism did not further increase GH mRNA levels above control, but increased PRL mRNA levels 2-fold over control. No changes were seen in FSH β, LH β, or POMC mRNA levels with any treatment. Thus, studies of altered thyroid status in the rat reveal dose-response and timecourse variability in the pretranslational control of TSH β, asubunit, GH, and PRL by thyroid hormone.