Thyroid hormone (T3 and T4) down-regulation of TSH subunit steady state mRNA levels and subunit gene transcription in vitro and in vivo has been well studied. We present evidence here that T3 can also regulate the turnover of TSH subunit mRNA. The apparent half-life of the TSH beta-subunit mRNA was determined by adding actinomycin-D (2 microM) to dispersed rat pituitary cultures in hypothyroid medium or medium containing 10(-7) M T3 and analyzing the decline in subunit mRNA levels with time. The half-life of the TSH beta mRNA from those cultures treated with T3 was shorter than that of the control cultures (9 vs. greater than 24 h, respectively). A possible mechanism by which TSH beta-subunit mRNA stability is altered is through a change in the size of each mRNA's poly(A) tail. Northern blot analysis of total RNA from the above cultures revealed that T3 treatment reduces the size of the TSH beta-subunit mRNA. To determine if this alteration of mRNA size was due to a loss of a portion of the poly(A) tract and not to alternative splicing of the transcript or use of a secondary transcriptional start site, pooled RNAs were hybridized with oligo(dT) and subsequently digested with RNAse-H to remove the poly(A) tract. RNA blot analysis of these RNAs showed that T3 treatment results in the loss of most of the TSH beta poly(A) tail.(ABSTRACT TRUNCATED AT 250 WORDS)