Thyroid Disease in Women

The thyroid is a small, butterfly-shaped gland that sits at the front of the neck, just below the Adam's apple. Its job is to produce two hormones — thyroxine (T4) and tri-iodothyronine (T3) — that regulate how every cell in the body uses energy. They control metabolism, heart rate, body temperature, mood, digestion, and how quickly reflexes work. The output of the thyroid gland is itself regulated by TSH — thyroid-stimulating hormone — which is produced by the pituitary gland in the brain. When thyroid hormone levels fall, the pituitary produces more TSH to stimulate the thyroid; when they rise, TSH falls. This feedback system is the basis for thyroid function testing.

Thyroid disease falls into two broad categories: conditions in which the thyroid produces too little hormone — hypothyroidism (an underactive thyroid) — and conditions in which it produces too much — hyperthyroidism or thyrotoxicosis (an overactive thyroid). Both are common, both predominantly affect women, and both have well-established clinical pathways for diagnosis and treatment.^[1]

Hypothyroidism — the underactive thyroid — occurs in approximately 2% of the UK population, rising to more than 5% in those aged over 60. Women are 5 to 10 times more likely to be affected than men. The most common cause in the developed world is autoimmune Hashimoto's thyroiditis — a condition in which the immune system produces antibodies that progressively damage thyroid tissue, reducing its ability to make hormones. Long-term, untreated hypothyroidism is associated with cardiovascular disease, raised cholesterol, and other metabolic consequences.^[1]

Hyperthyroidism — the overactive thyroid, or thyrotoxicosis — affects approximately 2% of UK women and around 0.2% of men. In the UK, the most common cause is Graves' disease, an autoimmune condition that accounts for 60 to 80% of all thyrotoxicosis cases. It occurs predominantly in women aged 30 to 60 years. Untreated hyperthyroidism carries its own long-term risks, including increased cardiovascular morbidity (illness and complications caused by heart and blood vessel disease) and mortality, and bone loss leading to osteoporosis.^[1]

Both conditions can also occur in a milder, biochemical form — known as subclinical hypothyroidism and subclinical hyperthyroidism — where TSH levels are outside the normal range but T3 and T4 levels remain normal. These are often detected incidentally on routine blood tests, and whether to treat them depends on the degree of abnormality and whether symptoms are present.

Thyroid disease matters for several interconnected reasons. The first is scale: thyroid disorders affect around 1 in 20 people in the UK,^[3] making them one of the most common endocrine conditions in the country — and both hypothyroidism and hyperthyroidism disproportionately affect women across all age groups. The second is consequence: untreated or undertreated thyroid disease is not benign. Hypothyroidism, left unmanaged, raises cardiovascular risk and cholesterol. Hyperthyroidism, untreated, increases cardiovascular mortality and accelerates bone loss. These are not trivial long-term risks.

A particularly important clinical point for women is the overlap between thyroid symptoms and menopause. NICE NG145 explicitly notes that in menopausal women, symptoms of thyroid dysfunction — fatigue, weight change, mood disturbance, sleep disruption — may be mistaken for menopause and go untested.^[1] This matters clinically: a woman with undiagnosed hypothyroidism whose symptoms are attributed to menopause will remain undertreated, often for years. Thyroid function testing is straightforward — a single blood test — and should be considered when symptoms are consistent with either condition.

The symptoms of hypothyroidism and hyperthyroidism are broadly opposite in character, though both produce real effects on quality of life:

Both conditions also carry implications for pregnancy. Women with hypothyroidism require careful monitoring and often increased doses of Eltroxin (levothyroxine) during pregnancy. Women with hyperthyroidism need specialist management of their antithyroid medication, since some drugs cross the placenta and can affect the developing baby. Thyroid status is a recognised component of pre-conception and antenatal care for women with known thyroid disease.^[2]

One further dimension that is often overlooked is the emotional and social burden of thyroid disease. The symptoms of hypothyroidism — persistent fatigue, weight gain, brain fog, low mood — are frequently dismissed or misattributed, and many women report lengthy delays between symptom onset and diagnosis. Understanding that these symptoms have a physiological cause, and that effective treatment exists, carries genuine clinical value.

Investigation — starting with TSH

The first-line investigation for suspected thyroid dysfunction is a blood test measuring TSH — thyroid-stimulating hormone.^[1] TSH is the most sensitive early indicator of thyroid status: it rises when the thyroid is underperforming (hypothyroidism) and falls when it is overperforming (hyperthyroidism). NICE NG145 recommends:

• •If TSH is above the reference range → measure free T4 (FT4) in the same sample to confirm and assess the degree of hypothyroidism
• •If TSH is below the reference range → measure FT4 and free T3 (FT3) in the same sample to assess the degree of hyperthyroidism

For people with confirmed hypothyroidism, NICE recommends considering TPO antibody testing (thyroid peroxidase antibodies — proteins produced by the immune system that attack the thyroid) to establish whether the cause is autoimmune. Their presence confirms Hashimoto's thyroiditis. NICE also recommends awareness that high biotin intake from dietary supplements can produce falsely abnormal thyroid function test results — patients should be asked about this before testing.^[1]

NICE NG145 notes that thyroid dysfunction testing should be considered in people with type 1 diabetes or other autoimmune conditions, in those with new-onset atrial fibrillation (a heart rhythm disorder), and in those presenting with depression or unexplained anxiety — as thyroid dysfunction can present through these pathways. In menopausal women, symptoms of thyroid dysfunction may closely mimic those of menopause, and thyroid function testing is clinically appropriate where there is diagnostic uncertainty.^[1]

Managing hypothyroidism

The first-line treatment for primary hypothyroidism is Eltroxin (levothyroxine) — a synthetic version of thyroxine (T4) taken as a daily tablet.^[1] For most adults under 65 without cardiovascular disease, NICE recommends a starting dose of 1.6 micrograms per kilogram of body weight per day, rounded to the nearest 25 micrograms. For those aged 65 or over, or with a history of cardiovascular disease, a lower starting dose of 25 to 50 micrograms per day is recommended to minimise cardiovascular risk during dose titration.

Monitoring during dose titration is by TSH measurement every three months until two consecutive results within the reference range are achieved — after which annual monitoring is sufficient.^[1] NICE is explicit that TSH levels can take up to six months to normalise after starting treatment if TSH was very high or hypothyroidism was long-standing. Expecting rapid resolution of symptoms and adjusting doses prematurely on that basis is a recognised clinical error.

Two treatments are explicitly not recommended by NICE NG145:^[1]

• •Liothyronine (T3) — not recommended routinely, alone or combined with Eltroxin (levothyroxine). However, a small minority of people continue to have symptoms despite adequate Eltroxin (levothyroxine) treatment and normal blood results. For this group, a supervised trial of liothyronine combined with Eltroxin (levothyroxine) may be considered by a specialist endocrinologist — after other causes have been excluded and risks discussed fully
• •Natural thyroid extract (such as desiccated thyroid products) — not recommended; no UK marketing authorisation; insufficient evidence of benefit; long-term safety uncertain

Managing hyperthyroidism

Treatment for hyperthyroidism depends on its cause, severity, and the patient's individual circumstances. NICE NG145 sets out three main treatment options:^[1]

• •Antithyroid drugs — carbimazole or propylthiouracil. Tablets that suppress thyroid hormone production. Suitable for mild-to-moderate Graves' disease; requires regular blood monitoring. Side effects include risk of agranulocytosis (a serious drop in white blood cells — if a sore throat or infection develops, urgent blood testing is needed)
• •Radioactive iodine (I-131) — a single dose, taken by mouth, that selectively destroys overactive thyroid tissue. Non-invasive. Requires a period of limited close contact with others (typically a few weeks). Requires delay before pregnancy or fathering a child. May result in eventual hypothyroidism requiring Eltroxin (levothyroxine) long-term
• •Surgery (thyroidectomy) — removal of all or part of the thyroid. Offers rapid and definitive treatment; leaves a neck scar; may result in permanent hypothyroidism. Best option when the thyroid is significantly enlarged or causing compression symptoms

Thyroid disease and pregnancy

Thyroid disease in pregnancy requires careful management, as both inadequate and excessive treatment can have harmful consequences for the pregnancy and the developing baby.^[2]

For women with hypothyroidism who are pregnant or planning pregnancy, Eltroxin (levothyroxine) doses typically need to increase during pregnancy as thyroid hormone requirements rise. RCOG Green-top Guideline No. 76 (April 2025) recommends aiming to keep TSH at or below 2.5 mU/L (milliunits per litre) throughout pregnancy. Women with subclinical hypothyroidism who are known to be TPO antibody-positive, or whose hypothyroidism is identified in the first trimester, may also be offered Eltroxin (levothyroxine) to protect pregnancy outcomes.

For women with hyperthyroidism who are pregnant, RCOG GTG76 recommends propylthiouracil rather than carbimazole during the first trimester — because carbimazole is associated with a small risk of embryopathy (structural abnormalities in the developing baby) in early pregnancy. After the first trimester, the choice of antithyroid drug may be reviewed. TFTs (thyroid function tests) should be monitored every two to four weeks during the first half of pregnancy in women on antithyroid drugs, reducing to every four to eight weeks after 20 weeks.^[2]