Why You're Gaining Weight Even When Nothing Has Changed: Could It Be Your Thyroid?

Have you ever felt like your body just isn’t playing by the rules anymore?

You’re eating the same, maybe even exercising more, but the weight just won’t budge. You feel tired all the time, struggle with brain fog, and your moods are all over the place. If this sounds familiar, it might be more than “just getting older.” It could be your thyroid.

Let’s explore how low thyroid function—often overlooked or inadequately treated—can silently sabotage your weight, mood, and energy levels.

What Is the Thyroid and Why Does It Matter?

Your thyroid is a small, butterfly-shaped gland in your neck, but don’t be fooled by its size—it plays a huge role in your metabolism, energy, and hormonal balance.

It produces hormones—primarily T4 (thyroxine) and T3 (triiodothyronine)—that regulate how your body uses energy. T4 is the inactive hormone that needs to be converted into T3, the active form that does all the heavy lifting inside your cells: helping you burn fat, think clearly, and maintain a stable mood and body temperature.

When this system isn’t working well, everything slows down—including your metabolism.

Symptoms of Low Thyroid Function (Hypothyroidism)

Many women with low thyroid function don’t even know it. The symptoms can be vague and are often brushed off as stress or ageing. Here are some signs to watch out for:

• Persistent fatigue, even after a good night's sleep

• Weight gain or difficulty losing weight despite no dietary changes

• Brain fog, memory issues, or difficulty concentrating

• Feeling cold all the time, especially hands and feet

• Dry skin and thinning hair

• Constipation

• Low mood or anxiety

• Irregular or heavy periods

If these resonate with you, it’s worth taking a closer look at your thyroid health.

The Problem with Standard Thyroid Testing and Medication

Most GPs will test TSH (Thyroid Stimulating Hormone)—and sometimes T4—but rarely look at the full picture. If your TSH is “in range,” you’re told everything is fine. But many women still experience classic thyroid symptoms even when their TSH is normal.

Here’s the issue: TSH is only part of the puzzle.

• You might have enough T4, but your body may not be converting it into active T3 efficiently.

• You might have autoimmune thyroiditis (Hashimoto’s), which causes inflammation and fluctuating hormone levels, long before it shows up on standard tests.

• And crucially, the conversion of T4 to T3 requires key nutrients—like selenium, zinc, and iron.

The standard treatment? A prescription for Levothyroxine (synthetic T4)—but if your body can’t convert it into T3, you’re still going to feel unwell.

Why Nutritional Deficiencies Matter

Let’s go deeper.

If you’re low in certain nutrients, your thyroid function suffers. For example:

• Low Ferritin (stored iron) is one of the most common reasons women struggle with thyroid function and conversion. If your ferritin is below ~70 ng/mL, your cells may not be getting the oxygen or energy they need.

• Selenium and zinc are vital for converting T4 to T3.

• B vitamins support energy production and adrenal health, which is closely tied to thyroid balance.

These deficiencies can all contribute to low thyroid symptoms—even if your blood tests are technically “normal.”

Why You Might Want to Consider Deeper Testing

If you’ve been told your thyroid is fine, but you don’t feel fine, it’s time to look deeper.

Private, comprehensive thyroid panels can include:

• Free T3 and Free T4

• Reverse T3 (which can block active T3)

• Thyroid antibodies (to detect autoimmunity)

• Ferritin, selenium, zinc, B12, and more

These tests provide a clearer picture of what’s really going on—and they can help uncover why you might be struggling with weight, energy, and mood.

What You Can Do About It

Supporting your thyroid isn’t just about taking a pill. It’s about understanding the root cause and addressing it holistically.

This includes:

• Optimising nutrition to support hormone conversion and reduce inflammation

• Supporting gut health, which affects nutrient absorption and immune balance

• Managing stress, which can suppress thyroid function

• Creating a realistic weight loss plan that works with your hormones—not against them

Ready to Dig Deeper?

If you're tired of being told "everything looks normal" when you know something isn't right, you're not alone—and you're not imagining things.

I offer private, comprehensive testing and personalised support through my hormone and weight loss programme for women over 40. We'll look beyond the basic tests, get to the root of your symptoms, and build a practical, science-backed plan to help you feel like yourself again.

Get in touch if you’d like to explore this further. You don’t have to accept fatigue, weight gain, and brain fog as your new normal.