3,5,3’-Triiodothyroacetic Acid (TRIAC) Effects on Pituitary Thyroid Regulation and on Peripheral Tissue Parameters

Triiodothyroacetic acid in health and disease

Thyroid hormone (TH) is crucial for development and metabolism of many tissues. The physiological relevance and therapeutic potential of TH analogs have gained attention in the field for many years. In particular, the relevance and use of 3,3',5-triiodothyroacetic acid (Triac, TA₃) has been explored over the last decades. Although TA₃ closely resembles the bioactive hormone T₃, differences in transmembrane transport and receptor isoform-specific transcriptional activation potency exist. For these reasons, the application of TA₃ as a treatment for resistance to TH (RTH) syndromes, especially MCT8 deficiency, is topic of ongoing research. This review is a summary of all currently available literature about the formation, metabolism, action and therapeutic applications of TA₃.

Thermogenic effect of triiodothyroacetic acid at low doses in rat adipose tissue without adverse side effects in the thyroid axis

Triiodothyroacetic acid (TRIAC) is a physiological product of triiodothyronine (T(3)) metabolism, with high affinity for T(3) nuclear receptors. Its interest stems from its potential thermogenic effects. Thus this work aimed 1) to clarify these thermogenic effects mediated by TRIAC vs. T(3) in vivo and 2) to determine whether they occurred predominantly in adipose tissues. To examine this, control rats were infused with equimolar T(3) or TRIAC doses (0.8 or 4 nmolx100 g body wt(-1) x day(-1)) or exposed for 48 h to cold. Both T(3) doses and only the highest TRIAC dose inhibited plasma and pituitary thyroid-stimulating hormone (TSH) and thyroxine (T(4)) in plasma and tissues. Interestingly, the lower TRIAC dose marginally inhibited plasma T(4). T(3) infusion increased plasma and tissue T(3) in a tissue-specific manner. The highest TRIAC dose increased TRIAC concentrations in plasma and tissues, decreasing plasma T(3). TRIAC concentrations in tissues were <10% those of T(3). Under cold exposure or high T(3) doses, TRIAC increased only in white adipose tissue (WAT). Remarkably, only the lower TRIAC dose activated thermogenesis, inducing ectopic uncoupling protein (UCP)-1 expression in WAT and maximal increases in UCP-1, UCP-2, and lipoprotein lipase (LPL) expression in brown adipose tissue (BAT), inhibiting UCP-2 in muscle and LPL in WAT. TRIAC, T(3), and cold exposure inhibited leptin secretion and mRNA in WAT. In summary, TRIAC, at low doses, induces thermogenic effects in adipose tissues without concomitant inhibition of TSH or hypothyroxinemia, suggesting a specific role regulating energy balance. This selective effect of TRIAC in adipose tissues might be considered a potential tool to increase energy metabolism.

Comparative efficacy and side effects of the treatment of euthyroid goiter with levo-thyroxine or triiodothyroacetic acid

Euthyroid goiter is usually treated with TSH-inhibitory doses of levo-T(4) (L-T(4)). Because triiodothyroacetic acid (TRIAC) decreases TSH levels, the following study was perfomed: 36 euthyroid goitrous female patients (no cancer or chronic thyroiditis) were randomized to TRIAC (19.6 micro g/kg) (n = 19) or L-T4 (1.7 microg/kg) (n = 17) treatment during 11 months. Goiter volume; lumbar and femoral bone mineral density; serum osteocalcin; deoxypyridinoline; TSH; free T(4); total, high-density lipoprotein, and low-density lipoprotein cholesterol; and triglycerides were measured before and after the study period. Student's t test and chi(2) analysis were performed. TSH values (microunits per milliliter) in the TRIAC and L-T(4) groups were: 1.91 +/- 0.6 (basal) and 0.180 +/- 0.1 (after) and 2.1 +/- 2.5 (basal) and 0.180 +/- 0.3 (after), respectively. Thyroid volume decreased 37.9 +/- 35.4% in the TRIAC patients and 14.5 +/- 39.5% in the L-T(4) group (P = 0.069). Forty-two percent of the goiters with TRIAC reduced more than 50% their initial volume vs. 17.7% with L-T(4) (P = 0.15). With TRIAC, patients experienced fewer side effects. No differences in the changes of bone mineral density, serum deoxypyridinoline, osteocalcin, or the lipid profile were observed between both groups. The present results show that TRIAC is more effective than L-T(4) in the reduction of goiter size, with comparable effects on peripheral parameters.