Unexplained Weight Gain in London – Thyroid, Vitamins or Hormones? What Tests Reveal

Reviewed by Dr Lauren Holland • Marylebone Diagnostic Centre
Results within 24 – 42 hours. Westminster residents receive 20% off.

Unexplained weight gain can feel confusing, especially when you keep a normal diet and activity level. Many London patients describe sudden changes in weight, bloating, slower digestion, fatigue or a feeling of heaviness. While lifestyle choices can contribute, weight gain is often linked to thyroid imbalance, hormonal shifts, vitamin deficiency, low iron, blood sugar instability or changes in metabolism.

Clinically, we see many people experiencing weight gain during stressful periods, after illness, or when vitamin levels drop – especially during the winter months in the UK. Blood testing is one of the fastest ways to understand whether the cause is hormonal, nutritional or metabolic. This guide explains the common reasons for unexplained weight gain and the relevant tests available at Marylebone Diagnostic Centre.

Why Am I Gaining Weight Without Changing My Diet?

Unexplained weight gain often comes from internal changes in the body.

Common causes include:

Hormonal causes

• Thyroid imbalance
• Perimenopause or menopause
• High cortisol from stress
• Insulin resistance

Nutritional causes

• Low vitamin D
• Low B12
• Low magnesium or zinc
• Iron deficiency

Metabolic causes

• Slow metabolism
• Fluid retention
• Poor sleep
• Blood sugar fluctuation

Identifying the cause is essential for effective management.

Symptoms That Often Come with Weight Gain

• Fatigue
• Hair thinning
• Bloating
• Cold hands and feet
• Mood changes
• Anxiety
• Dry skin
• Muscle weakness
• Heavy periods
• Low sex drive
• Brain fog

These symptoms usually indicate hormonal, thyroid or nutrient imbalance.

When is Weight Gain a Medical Concern?

You should seek testing if:

• Weight increases rapidly
• You gain weight without overeating
• Fatigue becomes severe
• You feel cold often
• You have swelling or puffiness
• Your menstrual cycle changes
• You have episodes of shakiness
• You feel persistently low or down

These symptoms suggest underlying metabolic or hormonal changes.

Risk Factors for Unexplained Weight Gain in London

You may be more likely to experience weight gain if you:

• Sleep less than 6 hours
• Have high stress levels
• Work long or irregular hours
• Spend long periods indoors (low vitamin D)
• Eat irregularly due to commuting
• Have a history of thyroid disease
• Recently changed contraception
• Are perimenopausal
• Drink alcohol regularly
• Have a family history of diabetes

Lifestyle patterns in London strongly influence metabolism.

Medical Causes of Weight Gain – What Blood Tests Reveal

At Marylebone Diagnostic Centre, several tests help uncover the cause.

Thyroid Panel

Thyroid imbalance slows metabolism.

Symptoms include:

• Weight gain
• Fatigue
• Cold sensitivity
• Dry skin
• Hair loss

Both underactive and overactive thyroid can cause metabolic shifts.

HbA1c (Blood Sugar Stability)

Blood sugar dysregulation leads to:

• Weight gain around the stomach
• Fatigue after meals
• Sugar cravings
• Energy crashes

Vitamin D

Low vitamin D is linked to:

• Weight gain
• Low mood
• Slow metabolism
• Poor muscle function

Common during winter and in office-based workers.

Vitamin B12 & Folate

Low B12 affects energy production and metabolism.

Symptoms include:

• Fatigue
• Dizziness
• Weakness
• Slow digestion

Ferritin (Iron Stores)

Low iron affects metabolism and causes:

• Weight changes
• Fatigue
• Breathlessness
• Hair thinning

Liver & Kidney Function

Useful when weight gain includes:

• Swelling
• Water retention
• Puffiness

Full Blood Count (FBC)

Checks for anaemia and immune activity.

Vitamin & Mineral Profile

Ideal when weight gain is combined with:

• Bloating
• Slow digestion
• Weakness
• Cravings
• Muscle fatigue

Which MDC Profiles Are Best for Weight Gain?

Wellness Profile

The most comprehensive option for unexplained weight gain.

Covers:

• Vitamin D
• B12
• FBC
• Liver & kidney function
• Blood sugar
• Minerals
• Ferritin

Thyroid Profile

Essential when weight, energy and mood are affected.

Vitamin & Mineral Profile

Best for slow metabolism and low energy.

HbA1c

Ideal for central weight gain and blood sugar instability.

Symptom-Test Matching Table

FAQs (People Also Ask)

Book Your Weight Gain Investigation at Marylebone Diagnostic Centre

Marylebone Diagnostic Centre
73 Baker Street, London W1U 6RD
Tel: +44 7495 970109
Open Monday–Saturday, 08:00–16:00

Fast, private and confidential testing with results in 24–42 hours.
Westminster residents receive 20% off.

Quick Links

• Advanced Sexual Health Panel
• STI Testing London
• Cardiac Profile
• HbA1c Test
• Full Lipid Panel
• Fatigue and Energy Profile
• Kidney Function Test
• Electrolytes Test
• Liver Function Test
• Thyroid Function Test
• Magnesium Test
• Full Blood Count
• Urinalysis & Culture
• Book an Appointment
• ECG Testing
• Wellness Profile
• Pancreatic Profile

Read More

• NHS – Unexpected Weight Gain
• British Thyroid Foundation – Thyroid and Weight
• NICE Guidelines – Obesity and Metabolic Assessment
• Harvard Health – Vitamin D and Weight
• Endocrine Society – Hormones and Weight