A Comprehensive Guide to Thyroid Health for Women MyOva

Underactive Thyroid: Symptoms, Tests and What Helps

You're tired no matter how much you sleep. You feel cold when everyone else is fine. Your hair is thinning, your periods have changed, and your head feels full of fog. Nobody has mentioned your thyroid.

Not the panic-inducing version of thyroid content. The "let's understand what's actually happening" version. Your thyroid is a small gland with a big job, and when it underperforms, the symptoms are easy to blame on stress, age or being busy.

You're not imagining it. Your body is trying to tell you something, and a simple blood test can usually tell you whether your thyroid is part of the picture. In the UK, doctors usually start with one measurement, TSH, rather than a long panel [1]. PCOS is now PMOS (polyendocrine metabolic ovarian syndrome), so this guide uses both names. Your diagnosis is a starting point, not a verdict.

This is what I wish someone had told me: how underactive thyroid is diagnosed and treated, how it overlaps with PMOS (PCOS), and what food and supplements can and can't do. Our guides to irregular periods and PMOS and the blood tests worth asking for are good companions.

A woman in a cream jumper sits at a wooden kitchen table by a sunlit window, holding a mug of tea and looking thoughtfully out, with an open notebook beside her

In this guide:

What Does the Thyroid Do?

In short: Your thyroid is a small gland in your neck that makes T4 and T3, hormones that set your metabolism's pace. The pituitary controls it using TSH.

The thyroid is a butterfly-shaped gland at the base of your neck. It makes two hormones, thyroxine (T4) and triiodothyronine (T3), which travel in your blood and influence nearly every cell, including how fast you burn energy, how warm you feel and how your gut moves.

Think of it as a volume dial rather than an on/off switch. The pituitary gland, a pea-sized structure at the base of the brain, releases thyroid-stimulating hormone (TSH) to tell the thyroid how much to make. If thyroid hormone drops, TSH rises to push for more. That feedback loop is why a raised TSH is usually the first sign of an underactive thyroid.

What Is Hypothyroidism?

In short: Hypothyroidism means your thyroid doesn't make enough hormone, so your body slows down. It's more common in women and usually develops gradually.

Hypothyroidism, or an underactive thyroid, means your thyroid isn't making enough T4 and T3. The NHS says symptoms usually develop slowly and get worse over time, and that the most common cause, Hashimoto's thyroiditis, is more common in women and usually starts between the ages of 30 and 50, although anyone can be affected [2]. NI Direct also says it's more common in women than men [3].

There's also subclinical hypothyroidism: a TSH slightly above the reference range with a normal T4. It may cause few or no symptoms, and whether it needs treatment depends on your TSH, your age and your symptoms (more on that below) [4, 1].

It creeps up over weeks, months or years, so many women adapt around it and assume they're just tired or stressed. Your body is trying to tell you something.

What Are the Symptoms of an Underactive Thyroid?

In short: Common symptoms include tiredness, weight gain, feeling cold, dry skin and hair, constipation, low mood, brain fog and irregular or heavy periods.

This list comes from the NHS [5]. The challenge is that nearly every item overlaps with something else, and NICE reminds doctors that one symptom alone may not point to thyroid disease [1].

Symptom What it can look like Worth knowing
Tiredness Exhaustion that sleep doesn't fix Shared with many conditions, so look for a cluster of symptoms rather than one.
Weight gain Gaining weight without changing what you eat Usually modest and partly fluid. The British Thyroid Foundation, reproducing American Thyroid Association content, says treatment brings only a small weight loss in many people [6].
Feeling cold, dry skin and hair, constipation Cold when others are comfortable, rough skin, hair that sheds, sluggish digestion All listed by the NHS [5].
Difficulty concentrating and low mood Brain fog, flat mood Both are on the NHS list, and both overlap with PMOS (PCOS) and many other conditions, so they're easy to misattribute.
Irregular or heavy periods Cycles that change length or flow Listed by the NHS [5]. See the fertility section below.
Croaky voice Hoarseness Also listed by the NHS [5].

These symptoms also overlap with PMOS (PCOS) and perimenopause, which is why thyroid function gets missed. You're not imagining it, and you deserve a proper look, not a shrug.

How Is Thyroid Disease Diagnosed?

In short: A blood test diagnoses it. NICE says to consider TSH on its own first, then add free T4 if TSH is high. Tell your GP if you take biotin.

Diagnosis is by blood test, and it's more stepwise than people expect. Here's what NICE says [1].

Test What NICE says What to ask
TSH Consider measuring TSH alone first when pituitary disease isn't suspected. "Can we start with a TSH test?"
Free T4 (and free T3) Free T4 is added in the same sample if TSH is raised. Free T4 and free T3 are added if TSH is low. "Is my free T4 normal?"
Thyroid peroxidase antibodies (TPOAb) Consider for adults with TSH above the reference range; not repeated. "Does the cause look autoimmune?"
Biotin check NICE says to ask about biotin intake, because high amounts from supplements can give falsely high or low results. "Should I stop my biotin before the test?"

Two things are worth knowing. First, you may have heard to demand a "full thyroid panel". That isn't NICE's approach, which is to add tests depending on the TSH result. If you've been diagnosed and still feel unwell on treatment, NICE does support checking free T4 alongside TSH, and says changes to treatment may improve symptoms for some people even when results are in range, so ask for a review [1].

Second, biotin. The MHRA warned in 2023 that biotin can interfere with thyroid blood tests [7]. It's in many hair, skin and nails supplements and in some multivitamins. Tell your GP and the lab if you take it. We couldn't find a UK source giving a number of days to stop beforehand, so ask the lab what they advise.

Is There a Link Between PMOS (PCOS) and Thyroid Problems?

In short: Studies often find underactive thyroid more in women with PMOS (PCOS), but evidence is mixed. Guidelines advise a TSH test, not routine screening.

This is a genuine overlap that gets too little airtime, but the research is more mixed than headlines suggest.

A 2025 hospital study in Tunisia compared 161 women with PCOS and 75 without. Subclinical hypothyroidism was found in 14.9% versus 5.3%, and thyroid antibodies in 12.4% versus 2.7% [8]. It's a single-centre study with borderline statistics, so treat it as a signal. A 2025 meta-analysis of 29 studies and 5,765 women with PCOS estimated that 19.7% had subclinical hypothyroidism, but results varied widely and most studies had moderate or high risk of bias. It had no control group, so on its own it can't prove a higher risk [9]. An earlier meta-analysis of six studies that did compare groups found about 2.9 times the odds [10].

The antibody picture is less certain. A 2025 meta-analysis found that when groups were matched for TSH and BMI, the difference in antibody prevalence was no longer significant [11], and a large study of infertile women found almost identical rates (8.2% versus 8.4%) [12]. So "more common on average" is fair, and "PCOS causes thyroid autoimmunity" is not established.

A narrative review notes that subclinical hypothyroidism is associated with worse insulin resistance and cholesterol in PMOS, but says direct evidence that thyroid problems cause PMOS or that treating them improves it is lacking [13].

What do the guidelines say? The 2023 international guideline lists TSH, prolactin and 17-hydroxyprogesterone tests among those used to exclude other causes when diagnosing PCOS [14]. The NICE draft PMOS guideline likewise says to offer TSH, with prolactin, oestradiol, LH and FSH, to people aged 10 and over with irregular or absent cycles to rule out other causes [15]. Neither recommends ongoing thyroid screening, and the draft is still out for revision (final expected December 2026). Some researchers do argue for wider testing, so the practical move is simple: if you have PMOS (PCOS), ask whether your thyroid has ever been checked, especially if you're cold, constipated, exhausted or have heavy periods.

How Is Hypothyroidism Treated?

In short: The usual treatment is a daily levothyroxine tablet (synthetic T4), with the dose set by blood tests. Timing and other supplements matter.

Levothyroxine replaces the hormone your thyroid isn't making. The NHS says you'll usually need to take it for the rest of your life, and that most side effects happen when the dose is too high and ease once it's adjusted [16]. NICE says to consider measuring TSH every 3 months until it has stabilised, then once a year [1].

How to take it. The NHS says to take it on an empty stomach, 30 to 60 minutes before your first meal, at the same time each day, and to avoid tea or coffee in that window [16]. Indigestion remedies, calcium and iron can stop it being absorbed. The UK product information says to separate oral iron, calcium salts and antacids by 4 to 5 hours [17], and the British Thyroid Foundation says at least four hours apart for calcium, iron and multivitamin tablets [18]. It also advises a four-hour gap from soya products [19].

Subclinical hypothyroidism. NICE says to consider levothyroxine when TSH is 10 mIU/L or higher on two occasions three months apart. For under-65s with symptoms and a TSH above range but below 10, it allows a six-month trial [1].

Untreated risks. The NHS says untreated hypothyroidism can lead to heart problems (including coronary heart disease and heart failure) and osteoporosis, and, in rare cases, a life-threatening condition called myxoedema coma. It also raises the chance of complications in pregnancy [20]. With the right dose, most people feel much better.

Weight. Weight loss after treatment is usually small [6], so levothyroxine isn't a weight-loss tool.

Can Thyroid Problems Affect Periods and Fertility?

In short: Yes, an underactive thyroid can cause irregular or heavy periods. NICE only suggests thyroid tests for fertility problems if you also have symptoms.

The NHS lists irregular or heavy periods as a symptom of an underactive thyroid [5], and the British Thyroid Foundation advises letting your doctor know if you're planning a baby and ideally having a blood test before you conceive [21].

It's worth being precise about what the guidelines say. NICE's fertility guideline (NG257, March 2026) keeps its earlier advice to offer thyroid testing for people with possible fertility problems only if they have symptoms of thyroid disease [22]. The committee noted that treating subclinical hypothyroidism to improve fertility is a research question, not a proven option [22].

If you take levothyroxine and are planning a pregnancy, ask your GP or endocrinologist whether your dose needs reviewing. The NHS says untreated hypothyroidism raises the chance of serious pregnancy complications, so don't stop your tablets [20].

Which Foods Support a Healthy Thyroid?

In short: A varied diet with iodine, selenium and zinc gives your thyroid what it needs. Most UK women get this from food, though vegans may need to plan.

Your thyroid needs iodine to make T3 and T4, and selenium (and iron) for the enzymes involved [23, 24]. Deficiency impairs thyroid function, but that research mostly comes from regions with severe deficiency [23, 25]. A 2011 survey of UK girls aged 14 to 15 found some degree of iodine deficiency in about two-thirds [26], so intake is worth thinking about, especially if you avoid dairy and fish.

Nutrient UK guidance and sources Watch out for
Iodine The NHS says adults need 140 µg a day. Sources: cows' milk, dairy, eggs, sea fish and shellfish. Some plant drinks are fortified [27]. If you avoid fish, eggs and dairy, the NHS suggests fortified foods or a supplement, with professional advice. Supplements of 0.5 mg a day or less are unlikely to cause harm, but higher long-term doses could change how your thyroid works [27].
Selenium The NHS gives 60 µg a day for women. Sources include Brazil nuts, fish, meat and eggs [28]. Brazil nut content varies widely. The British Dietetic Association says 5 to 6 nuts give 64 to 76 µg [29]. The NHS says 350 µg a day or less from supplements is unlikely to cause harm [28].
Zinc The NHS gives 7 mg a day for women. Sources include meat, shellfish, dairy, bread and cereals [28]. It helps regulate enzymes that handle thyroid hormones [30]. The NHS says not to take more than 25 mg a day from supplements unless advised by a doctor, because too much can reduce copper absorption [28].

Foods to be mindful of.

Brassicas (kale, cabbage, Brussels sprouts, cauliflower, turnips) contain goitrogens. The British Thyroid Foundation says you'd need to eat a large amount before it becomes a concern [19], and the Linus Pauling Institute says they matter mainly in large amounts or alongside iodine deficiency [31]. You don't need to avoid them.

Soya and fibre can affect levothyroxine absorption for some people. The British Thyroid Foundation suggests keeping soya at least four hours from your tablet [19]. We couldn't find a UK source on fibre, so ask your pharmacist if you eat a lot of it.

Kelp and seaweed. The British Dietetic Association says brown seaweeds such as kelp have an extremely high iodine content and shouldn't be eaten more than once a week [32]. The British Thyroid Foundation says people with an existing thyroid condition should avoid iodine supplements and kelp [33].

Do You Need Supplements for Your Thyroid?

In short: Most people don't. Supplements can fill a diet gap, but selenium and iodine aren't proven thyroid treatments, and some interact with levothyroxine.

Hormonal literacy isn't complicated, it's just rarely taught. Part of it is knowing that supplements aren't a thyroid treatment. If you eat dairy, eggs and fish, you're likely covering iodine. If you're vegan, it's the one to check.

Selenium is where marketing runs ahead of the evidence. A Cochrane review found selenomethionine reduced thyroid antibodies in Hashimoto's, but its clinical relevance was unclear and the evidence to support or refute selenium was incomplete [34]. A 2023 review concluded the benefits appear clearest when correcting a documented deficiency [25], and NICE lists selenium for subclinical hypothyroidism as a research recommendation, which tells you the evidence isn't there yet [35]. It's also not a reason to exceed the 350 µg a day supplement ceiling [28].

Iodine is the one to be careful with. Supplements can overshoot, and the British Thyroid Foundation says people already taking levothyroxine don't need one [36].

If you take levothyroxine, check every supplement for three things: iron or calcium (keep at least four hours from your tablet), kelp or other iodine, and biotin (it can distort thyroid blood tests). Always speak to your GP or pharmacist before adding anything.

Do Sleep, Stress and Exercise Affect Your Thyroid?

In short: The evidence is thinner than wellness content suggests. Sleep, stress and exercise matter for how you feel, but none is a proven thyroid treatment.

Sleep. TSH release follows a daily rhythm, rising at night [37]. One study of people with raised TSH found poor sleep was more common than in those with normal TSH, and TSH normalised more often in those whose sleep improved [38]. That's observational, so the direction of cause isn't settled.

Stress. Excess cortisol, as in Cushing's syndrome, lowers TSH and thyroid hormones [37]. That's a disease, not everyday stress. A review of autoimmune thyroid disease and stress in women concluded there is no definitive evidence of a causal link [39]. We can't say stress causes thyroid disease, though managing it is worth doing anyway.

Exercise. A 2023 review of 10 studies in people with hypothyroidism found exercise appeared safe and improved physical and mental outcomes, but made no significant difference to TSH, free T3 or free T4 [40]. Move for how it makes you feel, not as a thyroid treatment.

Where Does MyOva Fit In?

In short: MyOva's Women's Multivitamin has iodine and selenium, which contribute to normal thyroid function. It isn't a treatment, check with your GP first.

MyOva makes the Women's Multivitamin, so you deserve the straight version. Each two-capsule serving provides 150 µg of iodine (from kelp), selenium, 14 mg of iron, zinc, vitamin D3 and 400 µg of folate, with a B-vitamin complex that includes biotin, plus 350 mg of myo-inositol.

Iodine and selenium each contribute to normal thyroid function, and iron contributes to the reduction of tiredness and fatigue and to normal energy-yielding metabolism. Myo-inositol and the plant ingredients have no authorised health claims.

MyOva Women's Multivitamin
Daily Food Supplement
Women's Multivitamin

Iodine and selenium contribute to normal thyroid function, and iron contributes to the reduction of tiredness and fatigue. Contains kelp iodine, iron and biotin: read the notes below first.

Three things to know before you take it.

1. If you have a thyroid condition, the British Thyroid Foundation advises avoiding iodine supplements and kelp unless a clinician advises otherwise [33]. Speak to your GP or pharmacist first.

2. If you take levothyroxine, the multivitamin contains iron, so keep it at least four hours from your tablet [18, 17].

3. It contains biotin, which can interfere with thyroid blood tests [7]. Tell your GP and the lab, and ask whether to pause it before a test.

Speak to your GP or pharmacist before taking supplements if you take other medicines, are pregnant or breastfeeding, or are trying to conceive.

When Should You See Your GP?

In short: See your GP if you think your thyroid is underactive. Take a symptom list and ask whether TSH has been checked, especially if you have PMOS (PCOS).

The NHS says to see a GP if you think you might have an underactive thyroid [2]. NICE advises testing when there's clinical suspicion, while noting that one symptom alone may not indicate thyroid disease [1]. So go with a cluster, not a single complaint: tiredness, feeling cold, constipation, low mood, weight change and heavy or irregular periods.

Tell them if you have PMOS (PCOS), another autoimmune condition, or are planning a pregnancy, and mention any biotin or iodine supplements. Ask which tests they'd start with and when to retest. Your diagnosis is a starting point, not a verdict.

Infographic titled Underactive Thyroid: What to Know and Ask, with five points: symptoms overlap with many other causes, ask your GP for a TSH test and tell them if you take biotin, if you have PMOS (PCOS) ask whether your thyroid has been checked, take levothyroxine before breakfast and keep iron and calcium at least 4 hours apart, and check kelp, iodine, iron and biotin supplements with your pharmacist

Frequently Asked Questions

What is the difference between hypothyroidism and hyperthyroidism?

Hypothyroidism is an underactive thyroid that makes too little hormone, slowing things down. Hyperthyroidism is the opposite. The NHS lists symptoms including anxiety, difficulty sleeping, sensitivity to heat, a fast or irregular heartbeat, trembling and weight loss despite increased appetite. Treatments include antithyroid drugs, beta blockers, radioactive iodine and sometimes surgery [41].

Can hypothyroidism cause weight gain?

It can contribute, but usually modestly, and much of it can be fluid. Weight loss after treatment is usually small, so levothyroxine alone is unlikely to resolve a significant weight concern [6].

Is hypothyroidism permanent?

Usually, yes. The NHS says you'll usually need to take levothyroxine for the rest of your life [16]. Your GP can tell you whether anything about your cause suggests otherwise.

Can I eat normally with hypothyroidism?

For the most part, yes. Watch the timing of your tablet around food and supplements, and avoid kelp or iodine supplements unless advised [16, 36].

Can I take a multivitamin with levothyroxine?

Ask your GP or pharmacist first. Multivitamins often contain iron or calcium, which can stop levothyroxine being absorbed, so the British Thyroid Foundation advises at least four hours between them [18]. Check the label for iodine and biotin too.

The Bottom Line

Your thyroid is a small gland that sets your body's pace, and an underactive one is common, treatable and easy to miss. Diagnosis starts with a TSH blood test, treatment is usually a daily levothyroxine tablet, and food, supplements and lifestyle can support you but can't replace treatment. If you have PMOS (PCOS), ask whether your thyroid has been checked.

Hormonal literacy isn't complicated, it's just rarely taught. Start with one thing: write down your symptoms and book a GP appointment to ask about a TSH test.

Related Reading

This article is for educational purposes only and does not constitute medical advice. Guidance details are accurate to the sources cited at the time of writing, and NICE's PMOS guideline is still in draft. MyOva products are food supplements, not medicines, and are not intended to diagnose, treat, cure or prevent any disease. If you think you may have a thyroid problem, or you take thyroid medication, speak to your GP or a qualified healthcare professional before making changes to your treatment or supplements.

References

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  3. NI Direct. Underactive thyroid (hypothyroidism). https://www.nidirect.gov.uk/articles/underactive-thyroid-hypothyroidism
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Leila Martyn

Leila Martyn

Leila is the founder of MyOva, a UK-based hormonal health brand for women with PMOS (PCOS), endometriosis, perimenopause, PMDD and fertility challenges. Drawing on lived experience and scientific research, she shares evidence-based guidance to help women understand their hormones and cycles.

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References