Does Mounjaro Lower Testosterone? UK Clinical Evidence

Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist licensed in the UK for type 2 diabetes mellitus, known for its significant weight loss effects. Some patients wonder whether Mounjaro lowers testosterone levels, particularly men concerned about hormonal changes during treatment. Current evidence suggests that Mounjaro does not directly suppress testosterone through its pharmacological action. In fact, the substantial weight loss achieved with tirzepatide may improve testosterone levels in men with obesity-related hypogonadism, rather than reduce them. Understanding the relationship between weight loss medications and hormonal health is essential for patients considering or currently taking Mounjaro.

Understanding Mounjaro and Hormonal Effects

Mounjaro (tirzepatide) is a dual glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) receptor agonist licensed in the UK for the treatment of type 2 diabetes mellitus. The Medicines and Healthcare products Regulatory Agency (MHRA) approved this medication based on its efficacy in improving glycaemic control and associated weight loss effects.

Tirzepatide works by mimicking the action of incretin hormones, which are released after eating. These hormones stimulate insulin secretion when blood glucose levels are elevated, suppress glucagon release, slow gastric emptying, and reduce appetite through central nervous system pathways. The resulting weight loss can be substantial—with clinical trials in obesity (SURMOUNT programme) demonstrating reductions of 15-20% of body weight, while the diabetes trials (SURPASS programme) typically showed more modest but still significant weight reduction.

The relationship between weight loss medications and testosterone is complex and often misunderstood. Whilst some patients express concern that Mounjaro might lower testosterone levels, the evidence suggests a more nuanced picture. Understanding how this medication interacts with hormonal pathways requires examining both the direct pharmacological effects of tirzepatide and the indirect metabolic consequences of weight reduction. This distinction is crucial for patients considering or currently taking Mounjaro, particularly men with pre-existing concerns about testosterone levels or those experiencing symptoms potentially related to hormonal changes.

Does Mounjaro Lower Testosterone Levels?

There is no official link or established mechanism by which Mounjaro directly lowers testosterone levels through its pharmacological action. The drug's primary targets—GIP and GLP-1 receptors—are not known to directly interfere with the hypothalamic-pituitary-gonadal axis, which regulates testosterone production in the testes. Clinical trials of tirzepatide have not identified testosterone suppression as a recognised adverse effect, and the Summary of Product Characteristics (SmPC) approved by the MHRA does not list reduced testosterone among the documented side effects.

In fact, current evidence suggests that Mounjaro may have a neutral or potentially beneficial effect on testosterone levels in certain patient populations. For men with obesity and type 2 diabetes—conditions frequently associated with low testosterone (hypogonadism)—the metabolic improvements and weight loss achieved with tirzepatide may support healthier testosterone levels rather than suppress them. This represents an important distinction from the misconception that weight loss medications inherently reduce sex hormones.

It is worth noting that during the initial months of treatment, patients may experience various side effects related to the medication's mechanism of action. Common adverse effects according to the UK SmPC include:

• Gastrointestinal symptoms: nausea, vomiting, diarrhoea, and constipation

• Reduced appetite: which contributes to weight loss but may affect energy levels

• Fatigue: particularly during the adjustment period

Some of these symptoms—particularly fatigue and reduced energy—might be mistakenly attributed to hormonal changes when they are actually related to caloric restriction, rapid weight loss, or direct medication effects. If you experience persistent fatigue, reduced libido, or other symptoms suggestive of hormonal imbalance whilst taking Mounjaro, it is important to discuss these with your GP rather than assuming they are caused by testosterone reduction.

If you suspect you are experiencing side effects from Mounjaro, you can report these through the MHRA Yellow Card scheme.

How Weight Loss Affects Testosterone

The relationship between body weight and testosterone is well-established in medical literature, particularly in men. Obesity is strongly associated with lower testosterone levels, a condition known as obesity-related hypogonadism. Excess adipose tissue, especially visceral fat, increases the activity of the enzyme aromatase, which converts testosterone to oestradiol (a form of oestrogen). This process effectively reduces circulating testosterone whilst simultaneously increasing oestrogen levels, creating a hormonal imbalance that can perpetuate weight gain and metabolic dysfunction.

Additionally, obesity is associated with insulin resistance, chronic inflammation, and elevated leptin levels—all of which can negatively impact the hypothalamic-pituitary-gonadal axis and suppress testosterone production. Men with a body mass index (BMI) over 30 kg/m² are significantly more likely to have testosterone levels below the normal reference range compared to men of healthy weight. This relationship is dose-dependent, meaning that greater degrees of obesity are associated with progressively lower testosterone levels.

When significant weight loss occurs—whether through lifestyle modification, bariatric surgery, or pharmacological intervention such as Mounjaro—testosterone levels in men typically increase rather than decrease. Multiple studies have demonstrated that sustained weight reduction of 5-10% or more can lead to meaningful improvements in testosterone levels, often bringing previously low levels back into the normal range. This improvement occurs because:

• Reduced adipose tissue decreases aromatase activity

• Improved insulin sensitivity supports healthier hormonal regulation

• Decreased systemic inflammation allows better hypothalamic-pituitary function

• Lower leptin levels remove inhibitory signals on testosterone production

For women, the hormonal effects of weight loss can also be beneficial, particularly for those with polycystic ovary syndrome (PCOS), where weight reduction often improves hormonal balance and ovulatory function, as noted in NHS guidance. Therefore, the weight loss achieved with Mounjaro is more likely to improve rather than impair testosterone status in most patients.

Clinical Evidence and Research Findings

Clinical trial data from the SURPASS programme—the pivotal studies that led to Mounjaro's approval—provide reassurance regarding hormonal safety. These large-scale, randomised controlled trials involving thousands of patients with type 2 diabetes did not identify testosterone suppression as a safety concern. Whilst the primary endpoints focused on glycaemic control (HbA1c reduction) and weight loss, comprehensive safety monitoring would have detected significant hormonal disturbances had they occurred.

Research examining the effects of GLP-1 receptor agonists (a related class of medications) on testosterone has generally shown neutral or positive effects. A systematic review examining metabolic interventions in men with obesity and type 2 diabetes found that weight loss achieved through various methods, including incretin-based therapies, was associated with improvements in testosterone levels. Whilst tirzepatide is a newer agent with dual GIP/GLP-1 activity, the current evidence does not suggest it would negatively impact testosterone levels.

Observational data suggest that men with type 2 diabetes who achieve substantial weight loss with incretin-based therapies may experience:

• Improvements in total testosterone levels

• Improved sex hormone-binding globulin (SHBG) concentrations

• Better free testosterone (the biologically active fraction)

• Potential improvements in sexual function, likely related to improved metabolic health and vascular function

It is important to note that individual responses vary, and testosterone levels are influenced by numerous factors including age, baseline hormonal status, degree of weight loss achieved, and concurrent medical conditions. There are currently no specific recommendations from the National Institute for Health and Care Excellence (NICE) regarding routine testosterone monitoring for patients taking Mounjaro. However, if symptoms suggestive of hypogonadism develop or persist, appropriate investigation is warranted regardless of medication use.

When to Discuss Testosterone Concerns with Your Doctor

You should contact your GP or diabetes specialist if you experience symptoms that might suggest hormonal imbalance whilst taking Mounjaro. Symptoms potentially associated with low testosterone in men include:

• Persistent fatigue that does not improve after the initial adjustment period (typically the first few months)

• Reduced libido or sexual dysfunction that represents a change from baseline

• Mood changes, including low mood, irritability, or difficulty concentrating

• Loss of muscle mass beyond what would be expected with weight loss

• Reduced facial or body hair growth

• Gynaecomastia (breast tissue development in men)

It is important to recognise that many of these symptoms can have multiple causes. Fatigue, for example, might be related to caloric restriction, inadequate protein intake during weight loss, vitamin deficiencies (particularly vitamin D or B12), thyroid dysfunction, or simply the adjustment period to a new medication. Your doctor will take a comprehensive history and may arrange appropriate investigations.

Investigations that may be considered include:

• Testosterone levels: ideally measured in the morning (8-10 am) when levels are highest, with repeat testing on a separate day if initial results are abnormal. In the UK, total testosterone below 8 nmol/L is generally consistent with hypogonadism, while 8-12 nmol/L is considered borderline

• Sex hormone-binding globulin (SHBG): to calculate free testosterone

• Luteinising hormone (LH) and follicle-stimulating hormone (FSH): to assess pituitary function

• Prolactin: to exclude hyperprolactinaemia

• Thyroid function tests: as thyroid disorders can mimic hypogonadal symptoms

• Full blood count and metabolic panel: to exclude anaemia or other metabolic causes of fatigue

Urgent referral is warranted if you experience headaches, visual disturbances, or breast discharge (galactorrhoea), which could suggest pituitary disease, or if you notice a testicular mass or pain.

UK clinical guidance recommends that testosterone deficiency should be diagnosed based on both biochemical evidence (consistently low testosterone levels on two separate occasions) and clinical symptoms. If genuine hypogonadism is identified, your doctor will determine whether this is related to obesity (which may improve with continued weight loss), a pre-existing condition, or requires specific treatment such as testosterone replacement therapy. In most cases, patients taking Mounjaro who achieve significant weight loss are likely to experience improvements rather than deterioration in their hormonal profile, but individualised assessment remains essential for optimal care.

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