Hypothyroidism in women: an underactive thyroid, explained
An underactive thyroid is common in women and easy to miss. The symptoms, how it's diagnosed and treated, and where nutrition fits in.
Reviewed by Emma Carter, Senior Health & Nutrition Editor
An underactive thyroid slows your whole body down, and because it happens gradually, many women live with it for a while before it is found. It is common, far more frequent in women, and very treatable once diagnosed. Knowing what to look for, and what the treatment involves, helps you catch it early and get the most from it.
What hypothyroidism is
The thyroid, a small butterfly-shaped gland in your neck, sets your metabolic pace. It makes thyroid hormone, and your brain keeps it in check with a signal called TSH. When the thyroid is underactive it makes too little hormone, so the brain pushes TSH up to compensate, and everything in the body runs slower, from your energy and digestion to your heart rate and mood.
Symptoms that build slowly
The signs are easy to put down to a busy life, which is why they are so often missed:
- Fatigue and feeling run down
- Weight gain despite no change in eating
- Feeling cold, dry skin and hair thinning
- Low mood, constipation and poor concentration
- Heavier or irregular periods, muscle aches and a puffy face
Because these overlap with so much else, a blood test is the only reliable way to know.
Who gets it, and why
Most cases in women are Hashimoto's, an autoimmune condition where the immune system gradually attacks the thyroid. Your risk is higher if you have a family history of thyroid or autoimmune disease, another autoimmune condition, or have recently had a baby, since the thyroid can flare in the months after birth. An underactive thyroid can also follow surgery or treatment for an overactive thyroid, and some medicines affect thyroid function.
How it is diagnosed
A thyroid function test measures TSH and free T4. A high TSH with a low T4 confirms an underactive thyroid. A mildly high TSH with a normal T4 is called subclinical hypothyroidism, a gray zone that is sometimes just monitored and sometimes treated, depending on the level, your symptoms, and whether you are pregnant or trying to conceive. Your doctor may also test thyroid antibodies to check for Hashimoto's. Results are always read against your symptoms, not the number alone. Reference ranges are assay-dependent, with the upper limit usually around 4.2 to 4.5 mIU/L, and in people over 70 the normal upper limit can reach about 6.0, so age changes how a borderline result reads (Thyroid Research, 2015).
How it is treated
Hypothyroidism is treated with medication, usually levothyroxine, which replaces the hormone your thyroid cannot make. It is normally lifelong hormone replacement, with the starting dose worked out from your body weight (Cureus, 2023). The details matter: take it on an empty stomach, generally 30 to 60 minutes before breakfast, at the same time each day, and keep iron, calcium and coffee several hours apart, since they blunt absorption. Most women take it long term, with occasional blood tests to fine-tune the dose, and symptoms usually improve over several weeks rather than overnight. Sticking to the same brand where possible keeps your levels steady.
Thyroid, fertility and pregnancy
This is worth its own note. An undertreated thyroid can affect periods, ovulation and early pregnancy, so if you are trying to conceive it is worth having your levels checked and well controlled first. In pregnancy, the body needs more thyroid hormone, so the levothyroxine dose usually has to be increased, often early on. If you are pregnant or planning to be, tell your doctor so your thyroid can be monitored more closely.
Where nutrition fits
Diet and supplements do not treat an underactive thyroid, and it helps to be clear about that. What nutrition can do is support how you feel:
- Iron matters. Low iron can worsen thyroid symptoms, so your doctor may check ferritin and correct low iron.
- Correct confirmed shortfalls. Low vitamin D or B12 can add to fatigue and are worth ruling out.
- Mind the timing. Iron and calcium supplements block levothyroxine, so take them hours apart.
- Skip the iodine megadoses. Too much iodine, from high-dose supplements or kelp, can worsen thyroid problems rather than help. Be wary of products marketed for "thyroid support".
When to see a doctor
See your doctor if you have several of these symptoms, especially with a family history of thyroid or autoimmune disease:
- Ongoing tiredness, weight gain and feeling cold
- Dry skin, hair thinning and constipation
- Low mood, poor concentration or heavier periods
A simple blood test settles it, and treatment makes a real difference. If you are already on levothyroxine and still feel unwell, go back rather than putting up with it, since the dose may need adjusting.
Frequently asked questions
What are the symptoms of an underactive thyroid?
They develop slowly and are easy to miss: tiredness, weight gain, feeling cold, low mood, dry skin, constipation, hair thinning and heavier or irregular periods. Because these overlap with many other things, a blood test is the only reliable way to confirm it.
How is hypothyroidism diagnosed?
With a thyroid function blood test measuring TSH and free T4. A high TSH with a low T4 confirms an underactive thyroid. A mildly high TSH with a normal T4 is called subclinical hypothyroidism. Your doctor may also check thyroid antibodies to look for Hashimoto's.
What causes an underactive thyroid?
In most cases it is Hashimoto's, an autoimmune condition where the immune system attacks the thyroid, which is far more common in women. It can also follow pregnancy, thyroid surgery or treatment for an overactive thyroid, and some medicines.
How should I take levothyroxine?
On an empty stomach, usually 30 to 60 minutes before breakfast, at the same time each day. Keep iron and calcium supplements, and coffee, several hours apart, since they reduce how much you absorb. Consistency matters more than the exact time.
Can diet or supplements treat hypothyroidism?
No. An underactive thyroid is treated with medication, usually levothyroxine, not diet. Good nutrition supports overall health, and correcting low iron or vitamin D can help you feel better, but supplements do not replace thyroid treatment, and high-dose iodine can make things worse.
References
- Reference ranges for TSH and thyroid hormones · Thyroid Research, 2015
- Underactive thyroid (hypothyroidism) · NHS
- Epidemiology, types, causes, clinical presentation, diagnosis and treatment of hypothyroidism · PMC / NIH