Symptoms of underactive thyroid

What happens when your thyroid slows down

An underactive thyroid, also known as hypothyroidism, occurs when the thyroid gland doesn’t produce enough hormones to keep the body’s systems running smoothly. The thyroid is a small, butterfly-shaped gland at the base of your neck, but its role is far from small. It regulates energy, metabolism, mood, body temperature, heart rate, and more.

When it underperforms, your metabolism slows down. As a result, both physical and mental processes can feel sluggish. This imbalance can gradually affect nearly every organ system, often without you noticing right away. Many people dismiss early signs as stress, ageing, menopause, or lifestyle factors.

How an underactive thyroid shows up in daily life

An underactive thyroid can cause a wide range of symptoms. These may be mild at first but tend to worsen if left unaddressed. Common signs include:

• Persistent fatigue, even after a full night’s sleep
• Unexplained weight gain or difficulty losing weight
• Feeling cold all the time, especially in hands and feet
• Dry skin or brittle nails
• Constipation
• Low mood or signs of depression
• Brain fog or trouble concentrating
• Thinning hair or hair loss
• Menstrual changes or fertility issues
• Puffy face or swelling, particularly around the eyes
• Slower heart palpitations
• Hoarseness or a feeling of fullness in the neck

Not everyone experiences the same symptoms, and their severity can vary depending on the root cause and how long the thyroid has been underfunctioning.

Why your thyroid may not be working properly

The most common cause of an underactive thyroid is Hashimoto’s thyroiditis, an autoimmune condition where the immune system mistakenly attacks the thyroid gland. But that’s not the only trigger.

Other possible causes include:

Autoimmune factors

• Hashimoto’s thyroiditis
• Postpartum thyroiditis

Nutritional deficiencies

• Iodine deficiency (less common in the UK but still possible)
• Selenium or zinc deficiency
• Low levels of vitamin D or B12

Stress and adrenal dysfunction

• Chronic stress can affect the thyroid–adrenal axis
• High cortisol may suppress thyroid hormone production

Hormonal imbalances

• Perimenopause or menopause
• Oestrogen dominance
• PCOS (Polycystic Ovary Syndrome)

Medications or medical interventions

• Overuse of certain medications like lithium or amiodarone
• Post-thyroid surgery or radioiodine treatment

Understanding what’s behind your underactive thyroid is essential for truly effective care.

How we find out what’s really going on

Functional lab tests are key to understanding what’s really going on with your thyroid and what’s causing the dysfunction. While traditional screenings often only check TSH (thyroid-stimulating hormone), we recommend a full thyroid panel that includes:

• Free T4 and Free T3 – the main thyroid hormones
• Thyroid antibodies (TPOAb and TGAb) – to screen for autoimmunity
• Testing for heavy metals, nutritional deficiencies, gut and liver function.
• Cortisol function (morning, noon, afternoon and evening)

A full health assessment by a Functional Medicine Specialist helps identify deeper imbalances such as hormones, immune system, digestion, detoxification, energy and production that can affect thyroid function.

Daily habits that support your thyroid

Supporting your thyroid starts with daily habits and personalised guidance. Here are some ways to protect and support optimal thyroid function:

• Prioritise rest and manage stress with practices like mindfulness, yoga or breathing techniques
• Focus on whole foods rich in selenium (like Brazil nuts), iodine (like seaweed), and zinc (like pumpkin seeds)
• Avoid highly processed foods that may trigger inflammation or autoimmunity
• Minimise exposure to endocrine disruptors (like BPA in plastics or synthetic fragrances)
• Exercise gently and consistently, but don’t overdo it — overtraining can stress the thyroid
• Stay warm and support circulation, especially during colder months
• Track your symptoms and energy levels daily to spot patterns

Restoring balance beyond just medication

Supporting an underactive thyroid isn’t just about replacing hormones — it’s about restoring balance. For many, thyroid medication such as levothyroxine is prescribed to supplement hormone levels. However, this doesn’t always address why your thyroid became underactive in the first place.

A comprehensive, root-cause approach focuses on:

• Restoring nutrient balance through diet and supplementation
• Healing the gut if leaky gut or malabsorption is present
• Balancing blood sugar and insulin resistance
• Addressing autoimmune triggers and inflammation
• Improving detox pathways to support liver and hormonal health

Some people improve significantly through lifestyle adjustments alone, while others benefit from a combination of tailored support and medication.

How we help you get your energy and balance back

At Forbes.health, we take an integrative, root-cause approach to underactive thyroid concerns. Rather than masking symptoms, we aim to help your body restore its natural balance. Here’s how we support you:

• We uncover what’s driving your symptoms through advanced lab testing in the convenience of your home and a full health history review
• You’ll work with a Functional Medicine Specialist who creates a personalised plan — no one-size-fits-all
• We guide you step-by-step with nutrition, exercise, sleep, and stress support tailored to your lifestyle
• Ongoing check-ins and digital health tools keep you motivated and regular re-testing helps you monitor your progress
• Results you can understand mean you’re empowered to take control of your health

Our mission? To help you feel energised, balanced, and in tune with your body again.