Is It Perimenopause or Thyroid? How to Tell the Difference — and Why It Matters
By Rene Schliebs — Clinical Nutritionist, Medical Herbalist & Menopause Expert
There is one conversation I have more than almost any other in clinic.
A woman comes in. She's exhausted, gaining weight, her hair is thinning, she can't concentrate, she's anxious, her skin is dry, she's cold all the time. She's been to her GP. She's been told her thyroid is normal. She's been told she's probably just perimenopausal.
And then we look more carefully. And what we find is that it's often both.
Thyroid dysfunction and perimenopause are two of the most common hormonal conditions affecting women in their 40s and 50s, and they share so many symptoms that they are routinely confused, routinely missed, and routinely undertreated. Understanding the difference, and knowing how to get both properly investigated can change everything.
The symptom overlap
Both perimenopause and hypothyroidism (underactive thyroid) can cause:
Fatigue and low energy. Weight gain despite no change in diet or exercise.
Brain fog and poor concentration.
Mood changes, anxiety and depression.
Hair thinning or loss. Dry skin.
Sleep disruption.
Constipation.
Low libido.
Feeling cold.
The overlap is so significant that many women with undiagnosed thyroid disease are told they are simply perimenopausal, and many women with perimenopause have their thyroid dismissed as normal when a more thorough investigation would reveal dysfunction.
What's different about each
Perimenopause tends to cause:
Symptoms that fluctuate with your cycle — better some weeks, worse others.
Hot flushes and night sweats (though not always).
Irregular periods — shorter, longer, heavier, lighter.
Mood swings that track hormonally.
Vaginal dryness. S
Symptoms that have emerged in your late 30s or 40s.
Hypothyroidism tends to cause:
Symptoms that are more constant rather than cyclical.
Significant unexplained weight gain.
Severe constipation.
Very dry skin and hair.
A slowed heart rate and very low body temperature.
Puffiness — particularly around the face and eyes.
Eyebrow thinning (the outer third in particular).
Symptoms that can occur at any age.
Hashimoto's thyroiditis — the autoimmune form of hypothyroidism — can cause fluctuating symptoms that mimic perimenopause very closely, because antibody attacks on the thyroid cause periods of hyperthyroid activity (anxiety, heart racing, sweating) followed by hypothyroid periods (fatigue, weight gain, brain fog). This is particularly commonly missed.
Why standard thyroid testing is not enough
This is where I spend a lot of time educating my clients, and where a great deal of women fall through the gaps.
Standard GP thyroid testing checks TSH only — thyroid-stimulating hormone. The pituitary gland produces TSH to stimulate the thyroid. A normal TSH means the pituitary is not screaming at the thyroid to work harder. It does not necessarily mean the thyroid is producing adequate hormone, converting it properly, or that there is no autoimmune process underway.
To get the full picture you need:
TSH — yes, but as one piece of the puzzle not the whole answer.
Free T4 — the primary hormone produced by the thyroid.
Free T3 — the active form of thyroid hormone that your cells actually use. Many women have adequate T4 but poor conversion to T3; T3 determines how you feel. This conversion is impaired by stress, inflammation, low iron, low selenium and gut dysfunction — all extremely common in perimenopause.
Reverse T3 (rT3) — an inactive form of T3 that can accumulate under chronic stress and block active T3 from working. Elevated rT3 is a marker of significant physiological stress and explains why many women feel hypothyroid even with normal T4 levels.
TPOAb and TGAb (thyroid antibodies) — these identify Hashimoto's thyroiditis. A woman can have normal TSH, normal T4, and be producing antibodies that are actively destroying her thyroid tissue. She will feel terrible and be told her thyroid is fine. These antibodies must be tested.
What to say to your GP
"I'd like a comprehensive thyroid panel please — not just TSH. I'd like Free T3, Free T4, Reverse T3 and thyroid antibodies (TPOAb and TGAb) included. I've been experiencing symptoms that could indicate thyroid dysfunction and I want to rule it out properly."
Some GPs will push back on Free T3 and Reverse T3 — they are not always funded. You can request them as patient-funded tests — the cost is usually modest and the information is clinically valuable.
Also ask for ferritin alongside thyroid testing. Iron is essential for thyroid hormone conversion and production. Low ferritin, even without clinical anaemia, significantly impairs thyroid function and is one of the most common reasons women feel hypothyroid on paper despite adequate thyroid hormone levels.
What if it's both?
Very often it is. Perimenopause and thyroid dysfunction have a bidirectional relationship: declining estrogen affects thyroid hormone binding and metabolism, and thyroid dysfunction makes perimenopausal symptoms significantly worse.
The women I see who have been struggling the longest are almost always dealing with both, and often with iron deficiency and vitamin D deficiency layered on top. Getting the full picture, testing everything properly, and building a plan that addresses all of it simultaneously is the difference between marginal improvement and genuinely feeling well again.
This is exactly what I do in consultations.
👉 Book a virtual consultation — I will help you understand your results, identify what's being missed, and build a personalised plan that addresses the full picture.
Warmly, Rene xo.
Rene Schliebs is a Clinical Nutritionist and Medical Herbalist with over 20 years of experience. menopausenaturally.co.nz