Triac Reduces Serum TSH Without Decreasing Alpha and Beta TSH Messenger RNAS

Abstract

Triac, 3,5,3'-triiodothyroacetic acid, was administered at doses of 1, 3, 9 or 30 micrograms/100 g body weight to hypothyroid rats to determine its effects on TSH secretion and pituitary mRNA content. Triac caused a dose-dependent decrease in serum TSH 6 h after injection. Pituitary content of mRNA subunits either remained at hypothyroid levels or increased at 6 h. At 24 h after injection of the 3 micrograms dose of triac, serum TSH returned to hypothyroid levels; both alpha and beta mRNA subunits were reduced at this time. When 30 micrograms triac/100 g body weight was administered, serum TSH levels remained depressed 24 h later, while pituitary mRNA content was essentially the same as in the hypothyroid controls. These findings indicate that the initial decrease in TSH secretion in response to triac is independent of the availability of TSH mRNA transcripts and suggest that TSH secretion and synthesis may be differentially controlled.

Triiodothyroacetic acid in health and disease

“…From these and other studies it was estimated that 62 µg (100 nmol)/kg/day TA3 has an equal TSH-suppressive effect as 16 µg (20 nmol)/kg/day LT 4 . Interestingly, Mirell and coworkers (Mirell et al 1989) found that TSH mRNA expression levels are unchanged 6 h after TA 3 administration, suggesting that the initial decline in serum TSH is not caused by alterations at transcriptional level, but rather points to direct inhibition of TSH secretion by TA 3 . In contrast, prolonged (>12 days) TA 3 administration to rats persistently suppressed pituitary TSH mRNA levels (Juge-Aubry et al 1995, Liang et al 1997, resulting in a dose-dependent decrease in serum T 3 and T 4 levels (Medina-Gomez et al 2008).…”

Comparison of the Effects of 3,5,3'-Triiodothyroacetic Acid and Triiodothyronine on Goiter Prevention and Involution and on Hepatic and Skeletal Parameters in Rats

“…These results would indicate that although TSH is an important factor in thyroid cell proliferation, other TSH-independent signaling mechanisms may be of major importance to growth regulation of the thyroid gland [39]. It should be noted that other authors have previously demonstrated a prolonged effect on TSH inhibition after 30 g of TRIAC or 1 g of T 3 administered in a single daily dose [40].…”