Perimenopause vs Hypothyroidism Weight Gain: How to Tell Them Apart
Perimenopause vs hypothyroidism weight gain: symptoms overlap significantly. Learn the key differences and when to ask your GP for a thyroid test.
Two Common Causes of Unexplained Weight Gain
Weight gain that seems to arrive without explanation is one of the most frustrating aspects of midlife for many women. Two of the most commonly overlapping causes are perimenopause and hypothyroidism, an underactive thyroid. Both can cause weight gain, fatigue, low mood, brain fog, and feeling cold. Because they share so many features, distinguishing between them without testing is genuinely difficult. But the distinction matters, because the treatment is different.
How Perimenopause Causes Weight Gain
During perimenopause, declining oestrogen alters where the body stores fat, shifting distribution toward the abdomen. Falling oestrogen also affects insulin sensitivity, making the body more prone to storing calories as fat. Disrupted sleep raises cortisol and ghrelin, increasing appetite and reducing the body's tendency to burn fat efficiently. Muscle mass also tends to decline if resistance exercise is not prioritised. The result is weight gain that feels resistant to the strategies that worked in earlier decades.
How Hypothyroidism Causes Weight Gain
An underactive thyroid produces insufficient thyroid hormone, which slows the metabolic rate. This means the body burns fewer calories at rest. Weight gain from hypothyroidism tends to be moderate and is often accompanied by other distinctive signs: dry skin, hair loss or thinning, feeling unusually cold, constipation, slow heart rate, and a puffy face or swollen eyes. Fatigue in hypothyroidism tends to be pervasive and heavy rather than the fluctuating tiredness typical of perimenopause.
Symptoms That Overlap
Both conditions cause fatigue, low mood, brain fog, weight gain, and hair changes. This overlap is why many women with hypothyroidism are initially told it is perimenopause, and vice versa. Age complicates things further because both conditions become more common in the 40s and 50s. It is also worth noting that the two conditions can coexist; having perimenopause does not make you immune to developing thyroid problems.
How to Get Clarity
A simple blood test can measure TSH (thyroid-stimulating hormone) and, if needed, free T4 and T3. If your TSH is elevated and T4 is low, hypothyroidism is confirmed. If thyroid function is normal, perimenopause is likely the driving factor. Ask your GP for a thyroid function test if you have significant cold intolerance, unexplained constipation, very dry skin, or a particularly heavy and unrelenting fatigue. Hormone tests including FSH and oestradiol can also help confirm perimenopause status.
Managing Weight Gain in Either Case
Whether the cause is thyroid-related or hormonal, the foundational strategies overlap: prioritising protein intake, resistance training, reducing ultra-processed foods, and managing sleep. Hypothyroidism is treated with daily levothyroxine, which often resolves weight gain once the dose is optimised. Perimenopause-related weight gain responds well to consistent strength training, dietary adjustments, and sometimes HRT. Tracking your symptoms alongside lifestyle patterns in PeriPlan can help you build a clearer picture to share with your doctor.
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