Blood Tests for Weight Loss or Gain: Thyroid & Hormones

By Teck Geek | February 10, 2026

Sudden Weight Changes Without a Clear Reason

Experiencing weight changes can be unsettling, particularly when they happen without any obvious change in diet, activity level, or lifestyle. Many people find themselves gaining weight for no reason or losing weight without trying, and are left unsure where to turn for answers.

In these situations, a blood test for weight loss or unexplained weight gain can provide valuable insight into how the body is functioning beneath the surface. Weight regulation is closely linked to hormones, metabolism, blood sugar control, and thyroid activity. When one or more of these systems is out of balance, weight changes can occur even when calorie intake and exercise remain stable.

Common concerns include hormones and weight gain, hormones and weight loss, changes linked to stress, or difficulties related to blood sugar regulation. In some cases, people may also notice fatigue, changes in appetite, or fluctuations in mood alongside the weight change.

This article explains which blood tests for weight loss or weight gain may be helpful, and how markers such as thyroid hormones, insulin, glucose, and cortisol can contribute to unexpected changes in body weight.

Why Sudden Weight Changes Can Happen

When weight changes occur without clear changes to diet or activity, they are often driven by internal processes rather than lifestyle alone. At London Blood Tests, we frequently see people who are gaining weight for no reason or losing weight without trying, and are looking for objective answers rather than guesswork.

Body weight is regulated by a small number of key systems that can be assessed through targeted blood testing. These include thyroid activity, blood sugar and insulin balance, sex hormones, stress hormones, and metabolic processing by the liver. When one of these systems becomes disrupted, weight gain or weight loss may occur even when eating habits remain unchanged.

Hormonal imbalance is a common factor. Shifts in hormone levels can affect appetite, fat storage, and energy use, contributing to hormonal weight gain or hormonal weight loss. This is why hormone-related blood tests are often relevant when unexplained weight changes develop.

Thyroid function also plays a central role. The thyroid helps regulate how quickly the body uses energy. When thyroid activity is reduced or altered, it may lead to thyroid weight gain, difficulty losing weight, or in some cases weight loss with low thyroid function. Blood testing allows thyroid hormones and antibodies to be assessed clearly.

Blood sugar and insulin regulation are equally important. Changes in insulin response can promote fat storage or interfere with normal energy balance, which is why glucose and insulin blood tests for weight gain or weight loss are commonly considered in these situations.

Stress hormones and liver function also influence weight regulation. Elevated cortisol levels may encourage fat accumulation, while the liver plays a key role in processing hormones and glucose efficiently.

At London Blood Tests, these areas are assessed through focused blood tests and profiles, helping to identify potential imbalances and provide clearer insight into why sudden weight changes may be occurring.

Thyroid Function: A Key Driver of Weight Regulation

The thyroid plays a central role in controlling how the body uses energy. Even subtle changes in thyroid hormone activity can have a noticeable impact on body weight, which is why thyroid testing is often one of the first steps when investigating unexplained weight changes.

When thyroid hormone levels are reduced, metabolism can slow down. This may contribute to thyroid weight gain, difficulty losing weight, or the feeling of putting on weight thyroid issues despite no change in eating habits. Many people with reduced thyroid activity report fatigue, cold sensitivity, and gradual weight increase over time. In these cases, thyroid disease gain weight patterns are commonly explored through blood testing.

On the other hand, altered thyroid function can also contribute to unintended weight loss. Some individuals experience weight loss with low thyroid activity due to disrupted energy balance, appetite changes, or associated hormonal effects. Because symptoms can vary widely, blood tests are essential to assess thyroid function objectively rather than relying on symptoms alone.

Thyroid-related weight changes are not always caused by hormone levels alone. In some cases, immune-related thyroid activity plays a role, which is why antibody testing is often relevant when weight gain thyroid problems or unexplained weight loss persist.

Blood tests used to assess thyroid-related weight changes

At London Blood Tests, thyroid function can be evaluated using the following blood tests:

For a more comprehensive assessment, a Thyroid Full Profile brings these markers together, providing a broader view of thyroid activity and immune involvement.

Assessing thyroid function through blood testing helps clarify whether thyroid-related factors may be contributing to unexplained weight gain or weight loss, and provides useful information to support further clinical decisions.

Blood Sugar & Insulin Regulation

Blood sugar balance and insulin response are central to how the body manages weight. Even when eating habits remain the same, changes in this system can influence whether energy is used efficiently or stored as fat.

Insulin helps move glucose from the bloodstream into cells to be used for energy. When insulin regulation is disrupted, the body may store more energy than it needs, which can contribute to weight gain. This is why many people describe gaining weight on insulin or notice patterns associated with insulin gain weight, even without increased calorie intake.

In other situations, unstable blood sugar levels may affect appetite, energy levels, and metabolism, leading people to search for explanations around insulin and weight loss. While insulin is sometimes discussed in relation to weight reduction, imbalance rather than deficiency is more commonly involved. Assuming insulin for weight loss without testing can be misleading, which is why objective blood results are important.

Blood testing allows glucose and insulin levels to be assessed together, providing a clearer picture of how the body is handling energy. This combined approach is especially useful when investigating unexplained weight changes, as isolated results may not reflect the full metabolic picture.

Blood tests used to assess glucose and insulin balance

At London Blood Tests, insulin-related weight changes can be assessed using:

These tests are often reviewed together as part of glucose and insulin blood tests for weight gain or unexplained weight loss, helping to identify metabolic patterns that may contribute to difficulty maintaining a stable weight.

Lipid Profile & Metabolic Context

Changes in body weight often occur alongside shifts in how the body processes fats and energy. While cholesterol levels themselves do not directly cause weight gain or weight loss, they can reflect underlying metabolic changes that are happening at the same time.

When weight changes appear unexpectedly, lipid markers help provide additional context about metabolic health. Alterations in cholesterol or triglyceride levels may indicate changes in insulin sensitivity, hormone balance, or overall energy regulation. This is why lipid results are most useful when reviewed alongside thyroid, glucose, and hormone markers, rather than on their own.

For some individuals, unexplained weight gain may be accompanied by changes in lipid balance that reflect reduced metabolic efficiency. In others, unintended weight loss can also influence cholesterol patterns as the body adapts to altered energy demands. Looking at these markers together helps build a more complete picture of how the body is responding internally.

Blood tests used to assess lipid balance

At London Blood Tests, lipid status is assessed using:

These tests are commonly reviewed as part of a broader metabolic assessment, helping to place weight changes into a clearer physiological context.

Liver Function & Metabolic Processing

The liver plays a key role in how the body regulates weight, even though it is often overlooked when unexpected weight changes occur. It is involved in processing hormones, regulating blood sugar, and managing how energy is stored and released.

Hormones such as insulin, thyroid hormones, cortisol, and sex hormones are all metabolised by the liver. If liver function is not working optimally, these hormones may circulate for longer or behave unpredictably, which can influence appetite, fat storage, and energy levels. This can contribute to weight gain or weight loss even when diet and physical activity remain unchanged.

The liver is also central to glucose regulation. It helps balance blood sugar by storing and releasing glucose as needed. Disruption in this process can affect metabolic efficiency and may play a role in unexplained weight changes, particularly when insulin and glucose markers are already under strain.

For this reason, liver markers are best interpreted alongside thyroid, glucose, insulin, and hormone results rather than in isolation. Together, they provide a more complete picture of how the body is processing energy and regulating weight.

Blood tests used to assess liver function

At London Blood Tests, liver health is assessed using a Liver Function Tests package, which includes key markers involved in metabolic and hormonal processing.

Including liver function testing as part of a broader blood test assessment can help clarify whether metabolic processing is contributing to sudden or unexplained changes in body weight.

Sex Hormones & Weight Regulation

Sex hormones play a significant role in how the body distributes fat, regulates appetite, and maintains muscle mass. When hormone levels shift, weight changes can occur even without changes in diet or physical activity. This is why hormones and weight gain and hormones and weight loss are common concerns in people experiencing unexplained changes in body weight.

Hormonal imbalance may contribute to hormonal weight gain, particularly around the abdomen, hips, or thighs, and can also affect energy levels and motivation. In other cases, altered hormone levels may lead to hormonal weight loss, often accompanied by fatigue, changes in appetite, or reduced muscle mass.

These effects are not limited to one gender. Both male and female hormone fluctuations can influence weight regulation, body composition, and how the body responds to food and stress. Blood testing allows hormone levels to be assessed objectively, rather than relying on symptoms alone.

Female hormone considerations

Changes in female hormone balance can affect fluid retention, fat distribution, and metabolic rate. Hormonal shifts may contribute to difficulty losing weight or unexplained weight gain at different life stages.

Blood tests commonly reviewed include:

Male hormone considerations

In men, changes in testosterone availability can influence fat accumulation, muscle maintenance, and overall metabolic efficiency. Hormonal imbalance may contribute to weight gain or difficulty maintaining lean mass.

Blood tests commonly reviewed include:

At London Blood Tests, these hormone assessments are used to help clarify whether sex hormone imbalance may be contributing to unexpected weight changes, and to provide clearer data to support further medical decision-making.

Stress Hormones & Adrenal Function

Stress hormones, particularly cortisol, have a significant influence on how the body manages weight. Cortisol helps regulate energy use during periods of physical or emotional stress, but when levels remain elevated or become imbalanced, weight changes can occur.

Persistent stress is commonly associated with cortisol and weight gain, as higher cortisol levels can encourage fat storage, particularly around the abdomen. Many people experiencing cortisol weight gain notice increased appetite or difficulty losing weight despite no clear change in eating habits. This pattern, often described as cortisol gain weight, reflects hormonal influence rather than lifestyle alone.

In other cases, disrupted cortisol regulation may contribute to unintended weight loss. Altered cortisol patterns can affect appetite, muscle maintenance, and overall energy balance, leading to cortisol and weight loss or cortisol weight loss, especially during prolonged periods of stress or illness.

Blood testing provides an objective way to assess cortisol levels and understand how the body is responding to stress. Reviewing stress hormones alongside thyroid, insulin, and sex hormone markers helps identify whether adrenal imbalance may be contributing to unexpected weight changes.

Blood tests used to assess stress hormone balance

At London Blood Tests, stress-related hormonal balance can be assessed using:

These markers help provide insight into adrenal function and its potential role in unexplained weight gain or weight loss.

When Weight Changes Continue and You Need Clearer Answers

Many people seek testing because they are gaining weight for no reason or losing weight without trying, even after being reassured that initial results are normal. This can be confusing and discouraging, particularly when symptoms persist but no clear explanation has been given.

Weight changes are often influenced by overlapping factors such as thyroid weight gain, hormonal weight gain, or shifts related to hormones and weight loss. In some cases, people may also notice patterns linked to thyroid disease gain weight or difficulty losing weight alongside symptoms such as fatigue or low energy. Looking at related markers together is often more helpful than relying on a single result.

Blood testing can support clearer next steps, especially when paired with clinical guidance. At London Blood Tests, results can be reviewed as part of a broader assessment, and GP consultations are available to help interpret findings, discuss symptoms, and decide whether further investigation or monitoring is appropriate.

This approach is particularly helpful for those considering a weight loss blood test or broader blood tests for weight loss, as well as individuals exploring options for unexplained weight gain. Having both test results and access to medical advice can make it easier to move forward with confidence rather than uncertainty.

Take the Next Step

Unexplained changes in body weight can be frustrating, particularly when there has been no clear change in diet or activity. Factors such as thyroid weight gain, changes related to insulin gain weight, or shifts linked to hormones and weight loss can all influence how the body regulates weight.

A targeted blood test for weight loss or unexplained weight gain can help highlight potential hormonal, thyroid, or metabolic contributors. When results are reviewed together and interpreted in context, they can provide clearer insight and support more informed clinical decisions.

If you are gaining weight for no reason or losing weight without trying, blood tests for weight loss or weight gain may help clarify what is happening internally.

At London Blood Tests, we offer targeted blood testing alongside optional GP consultations, allowing results to be reviewed with medical guidance when needed. For those looking for blood tests for weight gain London private testing can provide earlier insight and support next steps without unnecessary delays.

You can book a blood test online or choose to add a GP consultation for further support with interpreting your results.