# Effects of _Aloe vera_ on the Regulation of Thyroxine Release in FRTL-5 Thyroid Cells
## 4 Discussion  
We demonstrated that AV regulates thyroid hormone production through the PKA pathway and TPO in FRTL-5 thyroid cells. Maintaining a constant level of hormones is a key factor in maintaining homeostasis in the body. Thyroid hormones are required for normal growth and development, and increase the basal metabolic rate of nearly every cell in the body. Therefore, an imbalance of thyroid hormones, such as through excessive or insufficient production in the body, may cause several symptoms reflecting metabolism. The production and secretion of thyroid hormones are highly regulated by complicated mechanisms in thyroid follicles located in the thyroid gland. This process involves several processes, including iodine transport into the cells by NIS, TG synthesis, iodination of the tyrosine on TG, conjugation of iodinated tyrosine residues by TPO, endocytosis, proteolysis of colloid containing iodinated TG, and secretion of thyroid hormones [16,17]. Each of these processes is enhanced by the binding of TSH to the TSH receptor. Stimulation of the TSH receptor by TSH activates G protein-coupled receptor effectors and signaling pathways such as the cAMP/PKA/ERK, cAMP/PKA/CREB, PKC/ERK, and PI3K-Akt pathways [15]. Of these pathways, the cAMP mainly plays a role in thyroid follicular cells and modulates the expression of thyroid-specific genes such as NIS, TG, and TPO [18]. In this study, AV increased and decreased thyroxine levels in media without (5H) and with TSH (6H), respectively (Figure 1). The effects of AV were not related to the number of cells (Figure 2); thus, AV may regulate the release of thyroxine in thyroid cells, and could be an effective candidate for affecting the role of TSH. Some studies have reported that herbal medicine can reverse the symptoms of hypothyroidism induced by propylthouracil (PTU) or L-thyroxine in rats. _Cuscuta Semen_ and Insamyangyoung-tang aqueous extracts were shown to cure PTU-induced hypothyroidism in rats [19, 20, 21, 22]. Similarly, Yangkyuksanhwatang, Palmulgunja-tang, Cheongpyesagan-tang, and Yukwooltang have been shown to effectively reverse hyperthyroidism induced by sodium levothyroxine in rats [21,22]. However, these herbal medicines have effects on either hypo- or hyperthyroidism, but not both, and differ from AV in this respect. In this study, we tested the ability of AV to regulate thyroxine release in two media conditions representing hypo- (5H) and hyperthyroidism (6H media) in FRTL-5 cells which are widely used to study thyroid function [23]. AV grows and is functional in media containing six hormones, including TSH. Therefore, an increase in thyroxine release in 5H media could compensate for the deficiency in thyroxin production, whereas a decrease in thyroxine release in 6H media could mean an excessive reduction in thyroxine production. AV may regulate thyroid function to maintain a constant level of thyroid hormones in the body and is, thus, a potential candidate for thyroid disease therapies. In clinical practice, _Aloe barbadensis_ Linne juice has been reported to improve [PERSON]'s thyroiditis-related subclinical hypothyroidism [24]. In male mice, AV decreased both triiodothyonine and thyroxine levels and could be effective for the treatment of mild hyperthyroidism [25]. However, there is a need for further research on AV-regulated thyroxine release in both hypothyroidism and hyperthyroidism animal models. We found that AV affected the mRNA and protein expression of TPO (Figure 3), which is a rate-limiting enzyme in thyroid hormone production and works on the iodination and conjugation of tyrosine on TG. In hyperthyroidism, anti-thyroid drugs such as PTU and methyliazole are known to interact with TPO to inhibit its function and decrease thyroid hormone synthesis [26]. It has also been reported that the anti-thyroid mechanisms of many compounds and flavonoids inhibit TPO activity in pigs [27]. AV may have similar characteristics to these chemical substances in terms of targeting TPO function in hyperthyroidism.