The thyroid is a small butterfly-shaped gland in your neck, and it sets the pace for almost everything your body does. It controls your metabolism, your energy, your temperature, your heart rate, your mood, and, in ways people rarely expect, your hair. When it runs too slow or too fast, the symptoms are wide-ranging and easy to put down to stress, age or a busy life. That is exactly why a thyroid problem is one of the most commonly missed causes of feeling unwell, and one of the most satisfying to find, because it is usually very treatable once identified.
If you have been tired, gaining or losing weight without explanation, feeling cold or anxious, or noticing your hair thinning, a simple thyroid blood test is the fastest way to know whether your thyroid is the reason.
What the thyroid actually does
Your thyroid produces two main hormones, thyroxine (T4) and triiodothyronine (T3). These circulate in your blood and tell nearly every cell how fast to work. The pituitary gland in your brain keeps them in balance using thyroid stimulating hormone (TSH): when thyroid levels drop, TSH rises to push the gland harder; when thyroid levels are high, TSH falls. That feedback loop is why TSH is the single most useful number in a thyroid panel, it reflects how hard your body is having to work to keep things steady.
Two things can go wrong, and they look almost like mirror images of each other.
An underactive thyroid (hypothyroidism)
This is the common one, particularly in women and with increasing age. The gland is not making enough hormone, so everything slows down. The classic pattern is:
- Persistent tiredness that sleep does not fix
- Weight gain, or difficulty losing weight despite no change in diet
- Feeling the cold when others are comfortable
- Dry skin, brittle nails, and hair thinning or shedding
- Constipation
- Low mood, brain fog, or poor concentration
- Heavier or irregular periods
None of these are specific on their own, which is the whole problem, they are the symptoms of ordinary modern life. Together, and especially when they build up over months, they point at the thyroid. The commonest cause of an underactive thyroid in the UK is Hashimoto’s, an autoimmune condition where the immune system gradually turns down the gland, which is why antibody testing matters (more on that below).
An overactive thyroid (hyperthyroidism)
Less common, but important to catch because it can strain the heart. Here the gland makes too much hormone and everything speeds up:
- Unexplained weight loss despite a normal or increased appetite
- A racing or pounding heart, or palpitations
- Feeling hot, sweaty, and restless
- Anxiety, irritability, or trouble sleeping
- Shaky hands (a fine tremor)
- More frequent bowel movements
- Hair thinning
Notice that hair loss appears on both lists. That is not a mistake. Any significant swing in thyroid function, in either direction, disrupts the hair growth cycle and pushes more follicles into shedding.
The thyroid and hair loss connection
This is the overlap that brings a lot of people to us. A thyroid imbalance is one of the most frequently missed causes of diffuse hair shedding, the kind where hair comes away from all over the scalp rather than in one patch. If you are testing because of hair shedding, thyroid function is one of the highest-yield checks in the whole workup, because it is common, easily corrected, and frequently sits alongside another fixable cause such as low iron.
That is why a proper hair panel never looks at the thyroid in isolation. Both the British Association of Dermatologists and NICE recommend checking ferritin and thyroid function as first-line investigations for unexplained hair loss, which is exactly what our hair panels do. If your shedding turns out to be thyroid-driven, correcting the thyroid usually settles the hair over the following few months. You can read more in our guide to female hair loss blood tests, where thyroid sits alongside ferritin as the two markers most worth getting right.
What a proper thyroid blood test checks
A single TSH is the standard NHS screen, and it is a reasonable starting point, but it does not tell the full story on its own. A thorough thyroid panel reads several markers together:
| Marker | What it tells you |
|---|---|
| TSH | The master signal. High TSH suggests an underactive thyroid, low TSH an overactive one |
| Free T4 | The main thyroid hormone available to your tissues, confirms and grades the problem |
| Free T3 | The active hormone, useful when symptoms and TSH do not quite match |
| Thyroid antibodies (TPO) | Flags Hashimoto’s or Graves’ autoimmune thyroid disease, the underlying cause in most cases |
At WMG Health this maps to our Hormone Specialist panel for a focused thyroid and hormone workup, or the General Wellness panel where thyroid is checked as part of a broader health screen. If hair loss is your main concern, the Hair Loss Essentials panel covers thyroid alongside the iron and vitamin markers that so often travel with it.
What “normal” really means
This is where reading results in context matters. Thyroid reference ranges are wide, and a TSH sitting near the top of the range with clear symptoms can be as relevant as one that is frankly abnormal. This is called subclinical hypothyroidism, and whether it needs treating depends on your symptoms, your antibodies, and your history, not on a single number in isolation. It is precisely the kind of result that a lab reporting only “normal” will gloss over.
Every thyroid result in our panels is reviewed by a GMC-registered doctor who reads your numbers against how you actually feel, so you get an explanation and a plan, not just a page of green ticks.
When to get tested
It is worth checking your thyroid if you have had any of the following for more than a few weeks:
- Unexplained fatigue, weight change, or feeling too cold or too hot
- Hair thinning or increased shedding, particularly alongside tiredness or heavier periods
- Palpitations, anxiety, or a tremor
- A family history of thyroid or autoimmune conditions
- You are planning a pregnancy, since thyroid balance matters for conception and early pregnancy
You do not need a GP referral. You can book a same-day thyroid blood test at our Harley Street clinic, with results reviewed by a GMC-registered doctor and a written explanation of what your numbers mean for you.
The takeaway
The thyroid is small, but it touches almost everything, energy, weight, mood, temperature and hair. Its symptoms are easy to dismiss because they overlap with everyday life, which is exactly why a blood test is the sensible way to rule it in or out. Check TSH, Free T4, Free T3 and antibodies together, read them against your symptoms, and you will know whether your thyroid is the cause, and if it is, it is very treatable.
Sources
- NICE Clinical Knowledge Summary: Hypothyroidism. cks.nice.org.uk
- NICE Clinical Knowledge Summary: Hyperthyroidism. cks.nice.org.uk
- British Thyroid Foundation: Your thyroid gland patient information. btf-thyroid.org
- British Association of Dermatologists: Telogen effluvium patient information leaflet. bad.org.uk
- NHS: Underactive thyroid (hypothyroidism) overview. nhs.uk
This article is for general information only and does not constitute clinical advice. If you have concerns about your symptoms, book an appointment or speak to your GP.