Hypothyroidism in Active People: Prevention, Diagnosis and When to See a Doctor

An underactive thyroid is one of the most common hormonal conditions in the UK, yet in sporty people it is easy to miss. Tiredness, heavy legs, weight creeping up and slow recovery are just as easily blamed on training load, poor sleep or a bad diet. Our editorial team explains how hypothyroidism develops, which tests matter and when it is time to book an appointment with your GP.

What hypothyroidism is

The thyroid is a butterfly-shaped gland at the front of the neck. It produces thyroxine (T4) and smaller amounts of triiodothyronine (T3), hormones that set the pace of metabolism in almost every tissue, from heart muscle to the brain. When the gland produces too little, the body effectively shifts into a lower gear.

In the UK and other iodine-sufficient countries, the most common cause is autoimmune thyroiditis, known as Hashimoto's disease. The immune system gradually damages thyroid tissue, and antibodies against thyroid peroxidase (TPO) are usually detectable. Other causes include previous thyroid surgery, radioactive iodine treatment and certain medicines such as amiodarone and lithium.

Women are affected several times more often than men, and the risk rises with age. A family history of thyroid disease, type 1 diabetes, coeliac disease or other autoimmune conditions also increases the likelihood. Pregnancy and the year after giving birth are periods of particular vulnerability.

Doctors distinguish overt hypothyroidism, where TSH is high and free T4 is low, from subclinical hypothyroidism, where TSH is raised but free T4 remains within the reference range. The second is far more common and does not always need treatment, which is a source of frequent confusion.

Symptoms and why athletes miss them

Classic symptoms develop slowly over months or years. Because the change is gradual, people often adapt to it and only realise how unwell they felt after treatment begins.

• Persistent fatigue and reduced exercise tolerance
• Feeling cold when others are comfortable
• Weight gain despite unchanged diet and training
• Constipation, dry skin, brittle hair or hair thinning
• Low mood, poor concentration, "brain fog"
• Muscle aches, cramps and stiffness; slow heart rate
• Heavier or irregular periods

Many of these overlap with overtraining syndrome and with relative energy deficiency in sport (RED-S). An athlete who has been underfuelling may develop a pattern sometimes called "low T3 syndrome", in which T3 falls as the body conserves energy. Research by Loucks and Heath showed that this occurs when energy availability drops below a certain threshold, even without any disease of the thyroid itself.

This distinction matters. Low T3 from underfuelling is an adaptation, and the answer is to eat enough, not to take thyroid hormone. True hypothyroidism, by contrast, is a problem of the gland and usually needs lifelong replacement therapy.

A further trap is self-medication. Some people in bodybuilding circles take thyroid hormones to accelerate fat loss. This can suppress the gland's own output, cause palpitations, bone loss and arrhythmia, and make later blood tests difficult to interpret.

How it is diagnosed

Diagnosis rests on blood tests, interpreted alongside symptoms. NICE guideline NG145 recommends testing TSH first, and measuring free T4 if TSH is outside the reference range. TPO antibodies help confirm an autoimmune cause.

Timing matters. TSH has a daily rhythm and can be temporarily affected by acute illness, so a single abnormal result is usually repeated after several weeks. Biotin supplements, common in hair and nail products, can interfere with some laboratory assays and should be stopped for a few days before testing, in line with lab advice.

Treatment for confirmed hypothyroidism in the UK is levothyroxine, a synthetic T4 prescribed and monitored by a GP or endocrinologist. The dose is adjusted according to repeat blood tests. Most athletes on stable replacement can train and compete normally; levothyroxine is not on the WADA Prohibited List.

Prevention and risk reduction

Autoimmune thyroid disease cannot be reliably prevented, because genetic factors play a major role. However, several modifiable factors influence thyroid health and the chance of developing problems that look like hypothyroidism.

Iodine is the raw material for thyroid hormones. In the UK, where table salt is not routinely iodised, milk, dairy products and white fish are the main sources. People following plant-based diets without suitable alternatives can fall short, a risk highlighted by Zimmermann and Boelaert. At the same time, very high iodine intake from kelp or seaweed supplements can itself trigger thyroid dysfunction.

Adequate energy intake is the second pillar. The 2023 IOC consensus statement on RED-S lists suppressed thyroid hormones among the endocrine consequences of chronic underfuelling. Matching food intake to training load protects not only the thyroid axis but also bone health, menstrual function and performance.

Finally, avoid using thyroid hormones or "thyroid support" products containing undeclared thyroid tissue. Analyses of such products have found active hormone content, which can cause symptoms of an overactive thyroid and confuse diagnosis.

When to see a doctor

Book a routine GP appointment if you have had several of the symptoms listed above for more than a few weeks, particularly if rest and adequate food intake have not helped. Mention any family history of thyroid disease or autoimmune conditions, and list all supplements you take.

Seek prompt medical attention if fatigue is accompanied by a noticeable swelling in the neck, difficulty swallowing, a hoarse voice that does not settle, or a very slow pulse with dizziness. These signs need assessment without delay.

Women planning pregnancy who have known thyroid disease, or symptoms suggestive of it, should discuss testing early, because thyroid hormone needs rise in pregnancy. Athletes already on levothyroxine should inform their doctor about major changes in training volume or body weight, as dose adjustments may be required.

Confusion, extreme drowsiness and low body temperature in someone with known hypothyroidism can signal myxoedema coma, a rare emergency. In this situation call 999 or go to A&E.

Conclusions of the editorial team

Hypothyroidism is common, especially in women, and its early symptoms closely resemble overtraining and underfuelling. A simple TSH blood test, with free T4 where needed, separates these possibilities.

Prevention focuses on what you can control: adequate iodine from food, enough energy to support training, and avoiding thyroid hormones or unregulated "thyroid boosters".

With correct diagnosis and monitored replacement therapy, most people with hypothyroidism return to full training and competition.

You may also find useful our articles on RED-S in male and female athletes, interpreting routine blood tests, and iodine on a plant-based diet.

References

1. Chaker L, Bianco AC, Jonklaas J, Peeters RP. Hypothyroidism. Lancet. 2017;390(10101):1550–1562.
2. National Institute for Health and Care Excellence. Thyroid disease: assessment and management (NG145). London: NICE; 2019.
3. Jonklaas J, Bianco AC, Bauer AJ, et al. Guidelines for the treatment of hypothyroidism: prepared by the American Thyroid Association task force on thyroid hormone replacement. Thyroid. 2014;24(12):1670–1751.
4. Loucks AB, Heath EM. Induction of low-T3 syndrome in exercising women occurs at a threshold of energy availability. Am J Physiol. 1994;266(3 Pt 2):R817–R823.
5. Zimmermann MB, Boelaert K. Iodine deficiency and thyroid disorders. Lancet Diabetes Endocrinol. 2015;3(4):286–295.
6. Mountjoy M, Ackerman KE, Bailey DM, et al. 2023 International Olympic Committee's (IOC) consensus statement on Relative Energy Deficiency in Sport (REDs). Br J Sports Med. 2023;57(17):1073–1097.