High or Low TSH in Athletes: Causes and What to Do Next
Thyroid-stimulating hormone (TSH) is one of the most frequently requested blood tests in the UK, and athletes see it on almost every routine panel. A result flagged as high or low can be unsettling, yet it is often explained by training load, diet, timing or supplements rather than thyroid disease. Our editorial team explains what TSH actually measures, why it moves in active people and when a result genuinely needs medical attention.
What TSH tells you and what it does not
TSH is produced by the pituitary gland and acts as the thermostat signal for the thyroid. When circulating thyroid hormones (free T4 and free T3) fall, the pituitary releases more TSH to push the gland harder; when they rise, TSH drops. Because of this inverse, roughly logarithmic relationship, a small shift in free T4 produces a much larger swing in TSH, which is why laboratories use it as the first-line screening test.
Most UK laboratories quote an adult reference interval of roughly 0.3–0.4 to 4.0–5.0 mU/L, but the exact limits differ by analyser and by trust. Always read your result against the range printed on your own report rather than one found online. A value marginally outside the interval is not automatically abnormal: by definition about 5% of healthy people fall outside any population reference range.
TSH alone cannot tell you why the system is out of balance. It needs to be interpreted with free T4, sometimes free T3, thyroid peroxidase (TPO) antibodies and, crucially, your symptoms and history. NICE guidance on thyroid disease (NG145) emphasises repeating borderline tests and not treating a single isolated number.
It is also worth knowing that TSH has a daily rhythm. Levels are highest at night and in the early morning and fall during the afternoon. A sample taken at 8 am and another at 4 pm can differ noticeably in the same person, which matters when comparing results over time.
Why TSH can be raised in athletes
A raised TSH with normal free T4 is called subclinical hypothyroidism. The commonest cause in the general population is autoimmune (Hashimoto's) thyroiditis, which is more frequent in women and runs in families. Athletes are not protected from it, so positive TPO antibodies alongside a creeping TSH deserve proper follow-up.
Several non-disease factors can push TSH up temporarily. Recovery from a recent illness is a classic example: during the recovery phase TSH may overshoot before settling. Iodine deficiency, still seen in parts of the UK where dairy and fish intake is low, can also raise TSH, particularly in plant-based athletes who do not use iodised salt or a suitable supplement.
Low energy availability is increasingly recognised as a driver of thyroid changes. The REDs (Relative Energy Deficiency in Sport) framework published by the International Olympic Committee describes a typical pattern of reduced T3 with low-normal TSH, but individual responses vary and some athletes show a mildly altered TSH as well. Certain medicines, including lithium and amiodarone, are also known to raise TSH.
Finally, laboratory interference should not be forgotten. Heterophile antibodies and very high-dose biotin can distort some immunoassays. Biotin is popular in hair-and-nail and "energy" products, and it can give falsely abnormal thyroid results on certain platforms.
Why TSH can be low
A suppressed TSH with raised free T4 or T3 suggests hyperthyroidism, most commonly Graves' disease or toxic nodular goitre. Symptoms such as palpitations, tremor, heat intolerance, weight loss despite good appetite and an unusually high resting heart rate should prompt a prompt GP appointment rather than waiting.
In the sporting world, a frequent cause of low TSH is exogenous thyroid hormone. Levothyroxine and liothyronine (T3) are sometimes misused for fat loss. This suppresses the pituitary signal, can cause arrhythmias and bone loss, and is a prescription-only practice that should never be undertaken without medical indication.
Other contributors include high-dose glucocorticoids, dopamine-agonist medicines, the first trimester of pregnancy and recovery after thyroiditis. Intense, prolonged endurance events can produce transient shifts, which is why a sample taken the morning after a marathon is a poor baseline.
Supraphysiological use of anabolic-androgenic steroids may also alter thyroid binding proteins and change total hormone values, while free hormone levels and TSH can move in less predictable ways. Anyone in that situation should disclose it honestly to their doctor so the results can be interpreted correctly.
Interpreting the combination of results
The table below summarises the common patterns. It is a simplified guide for orientation, not a diagnostic tool.
| TSH | Free T4 | Likely meaning | Typical next step |
|---|---|---|---|
| High | Low | Overt primary hypothyroidism | GP review, treatment discussion |
| Mildly high | Normal | Subclinical hypothyroidism or transient change | Repeat in 6–12 weeks, check TPO antibodies |
| Low | High | Hyperthyroidism | Urgent GP review, endocrinology referral |
| Mildly low | Normal | Subclinical hyperthyroidism, medication or recent illness | Repeat test, review medicines and supplements |
| Low or normal | Low | Possible pituitary cause or non-thyroidal illness | Specialist assessment |
Note how small the movement in free T4 needs to be to shift TSH considerably. This is why TSH reacts first and why borderline values should be confirmed before any conclusions are drawn.
A specialist will also look at trends. Two or three results taken under the same conditions are far more informative than one value taken after a hard week of training.
Practical steps if your result is out of range
If you are an athlete with an abnormal TSH, the following approach is sensible before anyone talks about treatment:
- Repeat the test in the morning, at a similar time, ideally after a rest day and not within a few days of a race or illness.
- Stop high-dose biotin supplements for at least two to three days beforehand, or as advised by the laboratory.
- Ask for free T4 and TPO antibodies alongside TSH if they were not included.
- Review your energy intake honestly; a sports dietitian can help identify low energy availability.
- Tell your GP about every medicine, supplement and performance-enhancing substance you use.
Iodine status is worth considering if you avoid dairy and seafood. The British Dietetic Association notes that milk is a major iodine source in the UK diet, so some plant-based drink users may fall short unless the product is fortified.
Do not self-treat with thyroid hormones or "thyroid support" blends containing undeclared hormone. Both can push the system into iatrogenic hyperthyroidism and make further testing much harder to interpret.
Seek urgent care if you experience chest pain, a very fast or irregular heartbeat, severe breathlessness or confusion, whatever your thyroid results show.
Conclusions of the editorial team
TSH is a sensitive and useful marker, but in athletes it is influenced by timing, recent illness, energy availability, supplements and medicines. A single out-of-range number rarely justifies alarm or treatment.
Persistently raised TSH with positive antibodies, or suppressed TSH with high thyroid hormones, are genuine medical findings that need a GP and often an endocrinologist. Exogenous thyroid hormone used for fat loss is a common and avoidable cause of suppressed TSH.
The best protection is a calm, structured approach: standardised repeat testing, a full panel and honest disclosure of everything you take.
For related reading, we suggest our pieces on interpreting a full blood count in athletes, on low energy availability and hormonal health, and on how supplements can interfere with laboratory tests.
References
- National Institute for Health and Care Excellence. Thyroid disease: assessment and management. NICE guideline NG145. London: NICE; 2019.
- Biondi B, Cappola AR, Cooper DS. Subclinical hypothyroidism: a review. JAMA. 2019;322(2):153–160.
- 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.
- Taylor PN, Albrecht D, Scholz A, et al. Global epidemiology of hyperthyroidism and hypothyroidism. Nat Rev Endocrinol. 2018;14(5):301–316.
- Li D, Radulescu A, Shrestha RT, et al. Association of biotin ingestion with performance of hormone and nonhormone assays in healthy adults. JAMA. 2017;318(12):1150–1160.
- 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.
