Effect of serum thyrotropin levels on the concentration of messenger RNA for thyroid peroxidase in the rat.

The effect of serum TSH on rat thyroid peroxidase mRNA levels was studied in order to investigate the regulation of thyroid peroxidase gene expression in vivo. A nearly full-length rat thyroid peroxidase cDNA clone was isolated from a bacteriophage cDNA library synthesized using poly A+ RNA isolated from the thyroids of propylthiouracil-treated rats. cDNA probes derived from this clone were used to study rat thyroid peroxidase mRNA levels in response to the level of serum TSH. Two major rat thyroid peroxidase mRNA bands were detected on Northern blots of total cellular RNA (at 3.2 kb and at 3.7 kb). Injection of thyroxine, which lowered the levels of serum TSH, also lowered the steady-state levels of both rat thyroid peroxidase mRNAs, whereas treatment with methimazole, which increased serum TSH, increased both rat thyroid peroxidase mRNA levels. In hypophysectomized rats 10 days postoperative, very low levels of thyroid peroxidase mRNA were observed. Injection of bovine TSH (1 IU/day) increased rat thyroid peroxidase mRNA expression, preferentially in the 3.2 kb band. These results clearly demonstrate that TSH regulates rat thyroid peroxidase mRNA levels in vivo.