Thyroid or perimenopause? How to tell the difference
Tiredness, brain fog, weight gain and low mood can come from your thyroid, perimenopause, or both. Here is how the symptoms compare and which tests help.
- Updated
- 4 min read
- By Charmaine Shepherd, IFM Certified Practitioner
An underactive thyroid and perimenopause share many symptoms, including tiredness, brain fog, weight gain, low mood and changes to periods. They also tend to start at the same stage of life, and many women have both. A thyroid blood test (TSH, ideally with T4 and antibodies) helps tell them apart. Your symptoms and cycle pattern tell us about perimenopause.
If you’re in your 40s, exhausted, foggy and gaining weight, you may have been told “it’s your hormones”. The question is which ones. Getting this right matters, because the treatments are completely different.
Why are thyroid problems and perimenopause so easy to confuse?
Both are common in women from their late 30s to early 50s. Hashimoto’s, the main cause of an underactive thyroid in the UK, usually starts between 30 and 50 (NHS). Perimenopause symptoms usually last 7 to 9 years, sometimes longer (NHS). The two can also affect each other, as changing oestrogen levels influence how thyroid hormone is carried and used.
How do the symptoms compare?
| Symptom | Underactive thyroid | Perimenopause |
|---|---|---|
| Tiredness | Very common | Common, often linked to poor sleep |
| Brain fog and poor concentration | Common | Common |
| Low mood | Common | Common, along with anxiety and mood swings |
| Weight gain | Common | Common, often around the middle |
| Periods | Can be heavy or irregular | Change in frequency and flow, then stop |
| Temperature | Feeling cold | Hot flushes and night sweats |
| Skin and hair | Dry skin, hair loss | Dry or itchy skin, hair thinning |
| Bowels | Constipation | Not typical |
| Voice | Can become hoarse | Not typical |
| Joints and muscles | Aches | Aches and joint pain |
| Vaginal and bladder | Not typical | Dryness, more UTI-like symptoms |
Sources: NHS underactive thyroid and NHS menopause.
Which clues point more one way?
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More likely thyroid: feeling cold when others are warm, constipation, a hoarse voice, a puffy face, a family history of thyroid or autoimmune disease.
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More likely perimenopause: hot flushes and night sweats, a changing cycle, symptoms that come and go with your cycle, vaginal dryness.
Plenty of women have features of both, so neither list is a diagnosis.
Which tests help?
| Test | Helps with | Notes |
|---|---|---|
| TSH | Thyroid | The standard first test |
| Free T4 (and free T3) | Thyroid | Shows how much hormone is available |
| TPO antibodies | Thyroid | Points to Hashimoto’s (British Thyroid Foundation) |
| Ferritin, B12, folate, vitamin D | Both | Low levels mimic both conditions |
| Full blood count | Both | Heavy periods can cause anaemia |
| FSH and oestradiol | Sometimes, perimenopause | Fluctuate a lot, so they’re not usually needed over 45 |
| Glucose or HbA1c | Both | Blood sugar changes add to fatigue and weight gain |
For most women over 45, perimenopause is diagnosed from symptoms rather than blood tests, because hormones swing so much from day to day. A thyroid test is worth doing if you have symptoms, because it’s simple and treatable.
What if it’s both?
It often is. Treating only one can leave you thinking “that didn’t work”. In clinic, I look at the whole picture: thyroid function, sex hormones, cortisol and stress, blood sugar, iron and nutrients, sleep and gut health. If you need levothyroxine or HRT, I’m happy to work alongside your GP. More in my perimenopause and menopause guide and thyroid guide.
When to see your GP
Contact your GP if your bleeding is getting heavier, or if you have palpitations. If you haven’t had a period for 12 months and then have any bleeding, see your GP (NHS). A lump in your neck or difficulty swallowing also needs prompt assessment.
Common questions
Can perimenopause affect thyroid test results?
Changing oestrogen levels, and starting or stopping HRT, can affect thyroid hormone binding. If you take levothyroxine, your dose may need checking when your HRT changes.
Can you have an underactive thyroid and perimenopause at the same time?
Yes, and it’s common, because both often begin between the late 30s and early 50s. Testing your thyroid helps make sure one isn’t hiding behind the other.
Do I need a blood test to diagnose perimenopause?
Usually not if you’re over 45, as symptoms and changes to your cycle often provide enough information. However, from a functional medicine perspective, testing can help us explore your individual hormonal picture. A blood test gives a snapshot of hormone levels, which can fluctuate considerably during perimenopause. In my practice, I often find urinary hormone metabolite testing more informative when exploring how your body processes hormones. This adds context to a personalised assessment rather than confirming perimenopause on its own. Blood tests remain useful for younger women, unclear presentations and checking other possible causes of symptoms.
Usually not, if you’re over 45. It’s diagnosed from symptoms and changes to your cycle. Blood tests may be used for younger women or when the picture is unclear.
Which symptom is the biggest clue?
Hot flushes and night sweats point strongly towards perimenopause. Feeling unusually cold, constipation and a hoarse voice point towards the thyroid.
Not sure which it is? Book a free discovery call.
Sources
Bloating in perimenopause? How your gut and hormones are connected
Hashimoto’s: what to do when your thyroid antibodies are raised
Brain fog in perimenopause: what’s normal, and what helps?
This article is for general information and isn't a substitute for personal medical advice. Please don't start or stop any medication without speaking to your GP or prescriber.