Thyroid: symptoms, values and which blood test gives insight

Your thyroid is a small butterfly-shaped gland in your neck that plays an outsized role in how you feel. It makes hormones that help steer your metabolism, energy, heart rate, weight and even your mood. When it runs too slow or too fast, you often notice vague symptoms first: tiredness, weight changes or a restless feeling. We often see people attribute these to something else for a long time, while a simple blood test can already point in a direction. A review in Nature Reviews Endocrinology estimates that a few percent of adults have some form of thyroid dysfunction, and part of it stays unnoticed (Taylor 2018).

This overview explains what your thyroid does, which symptoms may fit an underactive or overactive thyroid, and which values such as TSH and free T4 give insight. Think of it as the hub: the separate topics are worked out in their own articles below.

What does your thyroid do?

Your thyroid makes two main hormones: T4 (thyroxine) and T3 (triiodothyronine). Together they help set how fast your cells use energy. You can picture it as the accelerator of your metabolism.

The control comes from above. Your pituitary gland makes TSH (thyroid-stimulating hormone), which tells your thyroid how much T4 and T3 to produce. The system works with feedback, like a thermostat. A slow thyroid raises your TSH; a fast thyroid lowers it. That makes TSH an inverse indicator, and usually the first value a doctor checks.

Underactive or overactive: what is the difference?

Roughly, your thyroid can drift in two directions. An underactive thyroid (hypothyroidism) makes too little hormone, so many processes slow down. An overactive thyroid (hyperthyroidism) makes too much, and everything runs at a higher pace.

An underactive thyroid is more common. In the Netherlands the most frequent cause is the autoimmune condition Hashimoto's (Chaker 2017). For an overactive thyroid, Graves' disease is a common cause (De Leo 2016).

If the left column fits, read on about the underactive thyroid. If the right fits better, see the overactive thyroid.

Which symptoms may point to a thyroid problem?

Thyroid symptoms are often non-specific: they can fit many other things too, from lack of sleep to iron deficiency. That is exactly why a blood test helps: it takes the guesswork out.

• Persistent tiredness that does not lift with rest
• Weight change without a change in diet
• Hair loss, dry skin or brittle nails
• Feeling wired, or low and flat
• Trouble concentrating
• Feeling cold, or getting hot quickly

One symptom on its own says little. It is about the pattern, and whether symptoms persist. Your GP can help decide whether your thyroid plays a role.

Which blood values give insight into your thyroid?

A thyroid work-up almost always starts with TSH. If it is off, free T4 and sometimes free T3 and antibodies follow.

For what a specific number means, separate articles go deeper: read on about your TSH level, about TSH, T3 and T4 together, or about thyroid antibodies in Hashimoto's.

What do your thyroid values mean?

In short: a high TSH with a low free T4 fits an underactive thyroid, and a low TSH with a high free T4 an overactive one. The in-between forms are where it gets interesting.

A mildly raised TSH while free T4 is still normal is called subclinical hypothyroidism. Not everyone has symptoms and not everyone needs treatment (Biondi 2019). What is sensible depends on your symptoms, age and other factors. That is a conversation for your GP.

Your TSH can also be normal while you still have symptoms. Sometimes the cause lies elsewhere, for example in your iron or vitamin D.

Thyroid and fatigue, weight and mood

The three complaints people come to us with most are fatigue, weight change and a shift in mood. All three can relate to your thyroid, but none of them proves a thyroid problem on its own.

Read more in the articles on thyroid and fatigue and thyroid and weight. Broader fatigue causes are covered in the always tired overview.

When to have your thyroid tested?

There is no single rule for everyone. Some people choose to have their thyroid measured when symptoms persist, or when thyroid conditions run in the family. A test then gives a concrete starting point instead of guesswork.

A practical tip: have blood drawn in the morning, when TSH is highest, and pause biotin supplements for a few days beforehand, as biotin can disturb the measurement. More on preparation in the article on the thyroid blood test.

To measure your thyroid values without a referral, you can have a thyroid function test at Vitalcheck. Every result is reviewed by a BIG-registered doctor, so you see your result in context.

What can you do about abnormal values?

An abnormal value is not a diagnosis, but a signal. The next step is almost always a conversation with your GP, who combines your result with your symptoms. In our experience, people feel steadier walking into that conversation with a concrete number in hand. Discuss an abnormal result with your GP before changing anything yourself.

References

• Taylor PN, et al. Global epidemiology of hyperthyroidism and hypothyroidism. Nat Rev Endocrinol. 2018. PMID: 29569622.
• Chaker L, et al. Hypothyroidism. Lancet. 2017. PMID: 28336049.
• De Leo S, et al. Hyperthyroidism. Lancet. 2016. PMID: 27038492.
• Biondi B, Cooper DS. Subclinical Hypothyroidism: A Review. JAMA. 2019. PMID: 31287527.
• NHG-Standaard Schildklieraandoeningen. Dutch College of General Practitioners.

Every blood test result includes a professional assessment from a BIG-registered doctor. For treatment decisions, discuss your results with your GP.