Perimenopause and Thyroid Health: Symptoms, Testing, and What to Do

Why Thyroid Conditions Rise in Perimenopause

Thyroid conditions, particularly hypothyroidism (underactive thyroid), are significantly more common in women than men, and their prevalence rises with age. The perimenopausal years coincide with a period of increased autoimmune activity in many women, which raises the likelihood of developing Hashimoto's thyroiditis, the most common cause of hypothyroidism. Estrogen also influences thyroid hormone metabolism and the levels of thyroid-binding globulin (the protein that carries thyroid hormones in the blood), so hormonal fluctuations during perimenopause can affect thyroid function tests and how thyroid hormones are available to cells. This combination of factors means that if you are in perimenopause and experiencing unexplained fatigue, weight changes, mood shifts, or brain fog, thyroid function is worth checking even if prior results were normal.

Symptom Overlap: How to Tell the Conditions Apart

The symptom overlap between perimenopause and thyroid dysfunction is substantial and makes clinical assessment genuinely difficult. Hypothyroidism causes fatigue, weight gain, low mood, constipation, brain fog, dry skin, hair thinning, and feeling cold, all of which also occur in perimenopause. Hyperthyroidism (overactive thyroid) causes palpitations, anxiety, heat intolerance, weight loss, and insomnia, which again echo hot flashes and perimenopause-related anxiety and sleep disruption. Because the conditions can coexist and because one can mask or mimic the other, testing rather than clinical impression alone is essential. Keep a symptom log noting the pattern and timing of your symptoms. Hot flashes tend to be episodic and vasomotor in character, whereas thyroid-related symptoms are typically more persistent and do not come in waves. Presenting your symptom history clearly to your GP is the most useful thing you can do.

TSH Testing and What Results Mean

Thyroid-stimulating hormone (TSH) is the standard screening test. TSH is produced by the pituitary gland and rises when the thyroid is underperforming (signalling it to produce more) and falls when the thyroid is overproducing. Normal TSH reference ranges vary between labs but typically span from about 0.4 to 4.5 mIU/L. However, many clinicians working in thyroid and hormonal health argue that a TSH above 2.5 to 3.0 can be symptomatic in some individuals, particularly when thyroid antibodies are present. If your TSH is in the normal range but you have significant symptoms, asking for TSH plus free T4 and free T3 (the active thyroid hormones), and thyroid antibodies (TPO antibodies and TGAb), gives a fuller picture. Subclinical hypothyroidism (elevated TSH with normal free T4) is common and may be treated depending on symptoms, antibody status, and whether you are trying to conceive.

Hashimoto's Thyroiditis and Autoimmune Risk

Hashimoto's thyroiditis is an autoimmune condition in which the immune system attacks the thyroid gland. It is the most common cause of hypothyroidism in women in developed countries. The perimenopause transition appears to be a period of heightened autoimmune vulnerability, possibly related to the immunomodulatory effects of estrogen. Testing for TPO antibodies (thyroid peroxidase antibodies) identifies whether an autoimmune process is present. Many women with elevated antibodies have normal thyroid function for years before hypothyroidism develops, but the presence of antibodies warrants annual TSH monitoring. Anti-inflammatory dietary patterns, adequate selenium, and good sleep are practical steps that may support immune regulation, though they are not a replacement for medical management if thyroid function becomes impaired. If you have a first-degree relative with a thyroid condition, your own risk is elevated and proactive testing is particularly sensible.

Selenium, Iodine, and Thyroid Nutrition

The thyroid gland depends on specific nutrients to function. Iodine is the raw material for thyroid hormone production. Deficiency is rare in countries with iodised salt, but those following a strict whole-food plant-based diet with no dairy or fish may be at risk, as dairy and seafood are significant dietary sources. Selenium is essential for the conversion of T4 (the inactive thyroid hormone) to T3 (the active form) and for protecting the thyroid from oxidative damage. Low selenium is associated with elevated TPO antibodies. Brazil nuts (one to two per day) or a moderate selenium supplement (100 to 200 mcg) are commonly recommended. Avoid very high selenium doses, which can cause toxicity. Excessive iodine is also problematic for people with autoimmune thyroid disease, as it can trigger flares, so kelp or high-dose iodine supplements are best avoided unless specifically advised by your doctor. Zinc and iron deficiency can also impair thyroid hormone production and conversion, making a full micronutrient picture worth assessing.

Medication Adjustments During Perimenopause

Women already taking levothyroxine (the standard thyroid hormone replacement medication) may find their dose requirements change in perimenopause. Estrogen raises thyroid-binding globulin, which can bind more of the circulating thyroid hormone and reduce the free fraction available to cells. This means some women on HRT or experiencing estrogen fluctuations need a dose adjustment. Conversely, if oestrogen levels drop sharply, dose requirements can shift again. If you start or change HRT and notice a return of thyroid symptoms (fatigue, constipation, low mood, brain fog), ask your GP to recheck your TSH and free T4. Do not adjust your levothyroxine dose independently. Getting the dose right is a matter of gradual titration with blood test guidance, and symptoms alone are not enough to judge the correct dose accurately.

Monitoring and Working With Your Doctor