L-Thyroxine sodium salt pentahydrate, also known as Levothyroxine, is a synthetic hormone utilized in the study of hypothyroidism. It is biologically activated into Triiodothyronine (T3) from its original form, L-Thyroxine (T4), through conversion by DIO enzymes.

L-Thyroxine (T4) and Triiodothyronine (T3), two thyroid hormones, play crucial roles in regulating the expression of ion channels, pumps, and contractile proteins involved in muscle function. They significantly affect calcium homeostasis and its movement, essential for muscle excitation and contraction, by influencing their pharmacological control and secretion. The conversion of thyroxine (T4, a pro-hormone) to the more active form of thyroid hormone involves deiodinases; DIO1 and DIO2 enzymes activate thyroid hormone secretion, whereas DIO3 is involved in its inactivation. This conversion process is critical for regulating the secretion of thyroid-stimulating hormone (TSH) through a negative feedback mechanism. Experimental findings in rats have shown that a 12-week iodine-free diet leads to a marked reduction in both T3 and T4 levels compared to a control group on a standard diet (p<0.001). Treatment with low doses of L-Thyroxine results in an increase in T4 (p=0.02) without significant change in T3 levels (p=0.19). Conversely, high doses of L-Thyroxine elevate both T3 and T4 levels significantly in comparison to both hypothyroid and control groups (p<0.001 and p=0.004 for T3 and p=0.03 for T4, respectively).