Tesamorelin and IGF-1: How do they stimulate growth hormone pathways?

Tesamorelin stimulates growth hormone pathways by acting similarly to the body's natural growth hormone-releasing hormone (GHRH), which is responsible for signalling the pituitary gland to release growth hormone (GH). As GH levels increase, the liver and other tissues begin to produce higher amounts of insulin-like growth factor-1 (IGF-1). IGF-1 is an important hormone involved in metabolism, tissue repair, fat burning, muscle mass maintenance, and anabolic processes, which are the building and repairing of tissues. Unlike direct growth hormone injections, tesamorelin acts through the body's natural hormonal signalling system. This helps to maintain a more natural, pulsatile pattern of GH release while more closely controlling the rise in IGF-1 levels [1–5].

Structurally, tesamorelin is a stabilised synthetic version of human GHRH composed of 44 amino acids. It also includes a specific modification with trans-3-hexenoic acid which enhances stability and bioavailability, meaning it remains active for longer in the body and reaches target tissues more effectively [1,2]. Upon subcutaneous administration, tesamorelin binds to GHRH receptors located on somatotroph cells in the anterior pituitary gland. This activates intracellular cyclic AMP signalling and stimulates the rhythmic pulsatile release of endogenous GH. Growth hormone then primarily acts on the liver, where it activates signalling pathways that increase IGF-1 production. Elevated IGF-1 levels are responsible for many downstream effects, including increased fat burning (lipolysis), improved protein synthesis, enhanced mitochondrial activity, support of lean muscle mass, and regulation of glucose and lipid metabolism [1–5].

Clinical trials in humans have consistently shown that tesamorelin can significantly increase IGF-1 levels whilst improving metabolic health and body composition. In a placebo-controlled trial conducted by Falutz J et al. (2005), daily administration of tesamorelin increased IGF-1 levels by approximately 48% at a dose of 1 mg and by 65% at a dose of 2 mg after 12 weeks in individuals with HIV-associated abdominal fat accumulation [3]. These increases were associated with a marked reduction in visceral adipose tissue, improved triglyceride levels and an increase in lean body mass without any significant deterioration in glucose control. Another study by Falutz J et al. (2007) showed that tesamorelin increased IGF-1 whilst reducing visceral fat by approximately 15% and improving lipid-related metabolic markers in individuals with HIV-associated lipodystrophy [4].

Long-term studies have also confirmed sustained activation of the GH-IGF-1 pathway. In a pooled Phase III analysis by Falutz J et al. (2010), tesamorelin maintained elevated IGF-1 levels for up to 52 weeks, while continuing to reduce visceral fat and improve metabolic markers [5]. Stanley TL et al. (2012) further demonstrated that participants with greater visceral fat reduction also had improved adiponectin levels and stable glucose metabolism. Adiponectin is a hormone associated with insulin sensitivity and fat metabolism. These findings suggest that tesamorelin's stimulation of the GH-IGF-1 pathway may support broader metabolic improvements, rather than just causing isolated hormonal changes [6].

Increased IGF-1 levels also appear to be strongly linked to improved cellular energy production and mitochondrial function. Mitochondria are structures within cells responsible for energy generation. Makimura H et al. (2014) studied obese individuals with reduced GH secretion and found that the IGF-1 increase induced by tesamorelin was strongly correlated with improved phosphocreatine recovery and mitochondrial function, suggesting more efficient cellular energy production [7]. These findings suggest that activation of the GH–IGF-1 pathway may improve how the body utilises and produces energy, rather than simply reducing fat tissue.

The increase in IGF-1 associated with tesamorelin has also been linked to benefits for liver health. In clinical trials involving people with HIV-associated non-alcoholic fatty liver disease (NAFLD), tesamorelin significantly reduced the amount of fat in the liver, whilst improving gene expression and mitochondrial pathways associated with liver metabolism [8,9]. Fourman LT et al. (2020) demonstrated that tesamorelin increased the expression of genes involved in oxidative phosphorylation—a key process in cellular energy production—whilst reducing the activity of pathways associated with inflammation and liver fibrosis [8]. Researchers believe that these effects are partly related to GH and IGF-1 signalling, which influence fat metabolism, inflammation and the liver’s energy balance.

The GH–IGF-1 pathway activated by tesamorelin may also influence brain health and cognitive function. Baker LD et al. (2012) reported that tesamorelin improved executive function and certain aspects of memory in older adults with and without mild cognitive impairment, whilst increasing IGF-1 levels by approximately 117% within physiological norms [10]. Subsequent research by Friedman SD et al. (2013) suggested that hormonal changes were associated with beneficial alterations in brain chemistry, including increased GABA levels and reduced markers of nervous system inflammation [11]. GABA is a neurotransmitter responsible for calming brain activity and supporting the proper functioning of the nervous system. These results suggest that the increase in IGF-1 induced by tesamorelin may support neurological health in addition to metabolic functions.

A key difference between tesamorelin and direct growth hormone therapy is that tesamorelin stimulates the body's own GH production rather than providing external GH. This may help maintain a more natural hormonal regulation and potentially limit some of the metabolic side effects associated with excessively high GH levels. Clinical trials have repeatedly demonstrated that tesamorelin increased IGF-1 levels without significantly worsening fasting glucose or HbA1c in most participants [4,5,9]. However, because tesamorelin raises IGF-1 levels, regular monitoring is usually recommended during therapy, especially in those with diabetes risk factors or a history of cancer.

Generally speaking, tesamorelin stimulates growth hormone pathways via physiological activation of GHRH receptors in the pituitary gland, leading to increased endogenous GH release and higher IGF-1 production. This hormonal cascade contributes to reductions in visceral and hepatic fat, improvement in metabolic and mitochondrial function, maintenance or increase in lean body mass, and potentially supportive effects on cognitive function and inflammation. Current evidence suggests that tesamorelin activates the GH–IGF-1 axis in a clinically meaningful yet relatively controlled manner, distinguishing it from direct growth hormone administration.