Thyroid blood test: getting your thyroid tested

The thyroid is a butterfly-shaped organ at the base of the neck, just above the collarbone. It produces hormones that influence almost every tissue and organ: how fast your body uses energy, your body temperature, your heart rate, your digestion, your concentration and even your mood. That is why the symptoms of a thyroid out of balance are so broad.

The thyroid does not act on its own. The pituitary gland, a small gland beneath the brain, continuously measures how much thyroid hormone is circulating in the blood and adjusts it with the hormone TSH (thyroid-stimulating hormone). If there is too little thyroid hormone, the pituitary produces more TSH to spur the thyroid on. If there is too much, the pituitary eases off and TSH falls. This feedback loop is the reason TSH responds so sensitively: the pituitary notices a small shift before you do.

The hormone the thyroid mainly releases is T4 (thyroxine). The liver and other tissues convert T4 into T3, the form that does most of the work. Iodine is needed for production; both a shortage and an excess, for example from iodine-containing contrast fluid or supplements, can throw the thyroid off balance. Most of the T4 and T3 in the blood is bound to transport proteins; only the loose, free part is active. That is why the laboratory measures free T4 and, where needed, free T3, rather than the total.

An underactive thyroid (hypothyroidism) may relate to:

• fatigue
• weight gain
• feeling cold

An overactive thyroid (hyperthyroidism) relates instead to:

• restlessness
• palpitations
• weight loss

Symptoms are not specific, however, which is why blood testing is valuable. Below we look at those symptoms in more detail.

Thyroid symptoms often develop gradually and resemble ordinary busyness or getting older. With a thyroid that works too slowly, you may recognise the following:

• you are tired, even after a good night's sleep
• you feel cold easily
• you gain weight without eating differently
• your skin and hair are dry, and you lose more hair than usual
• your bowels are sluggish
• you feel low or think more slowly than usual

With a thyroid that works too fast, the symptoms look very different:

• you lose weight although you eat well
• you feel restless, agitated or easily irritated
• your heart beats fast or you have palpitations
• you sweat more than usual
• you sleep poorly and your hands tremble
• your bowels are faster, sometimes loose

Sometimes there is also a swelling in the neck, or eye symptoms appear. The important caveat every GP adds: none of these symptoms is specific. Fatigue also occurs with iron deficiency, lack of sleep, stress and low mood; weight change has dozens of possible causes. From the symptoms alone you cannot tell whether your thyroid is involved. Only a blood test shows whether the thyroid plays a part, and a result within range does not mean the symptom deserves no attention.

A thyroid blood test revolves around a small number of values that complement each other. What each of them tells you:

• TSH: the controlling hormone from the pituitary and the most sensitive first marker, because the pituitary adjusts before the thyroid hormone itself shifts clearly. A TSH that is too high usually means the pituitary has to pull hard on a thyroid that produces too little. A TSH that is too low usually means the pituitary is easing off because there is too much thyroid hormone.
• Free T4: the unbound part of the hormone the thyroid itself releases. Free T4 shows whether the thyroid is actually producing too little or too much, and how pronounced that is.
• Free T3: the active form that arises from T4 in the liver and other tissues. The laboratory measures free T3 as an addition when TSH and free T4 do not fit the symptoms, for example with signs of an overactive thyroid while the other values do not point that way.
• Anti-TPO: antibodies against thyroid peroxidase, a protein found only in the thyroid. Clearly raised antibodies fit an autoimmune cause, such as Hashimoto's disease in an underactive thyroid. A small amount of antibodies also occurs in healthy people and in other autoimmune conditions, and is not a diagnosis in itself.

Why TSH and free T4 together

TSH alone tells you whether the pituitary is satisfied; free T4 tells you what the thyroid is actually delivering. Only the combination gives a picture: a raised TSH with a free T4 still within range fits a mild or early slowdown; a raised TSH with a lowered free T4 fits a clearly underactive thyroid. For an overactive thyroid the same applies in mirror image. That is why the basic Bloodworks test always includes both values.

When TSH and free T4 are not enough

Sometimes the symptoms do not fit the result, or the doctor wants to know why the thyroid deviates. That is when free T3 and anti-TPO come into play. Free T3 helps when an overactive thyroid is suspected but not yet visible in TSH and free T4; anti-TPO helps to confirm or make unlikely an autoimmune cause, which matters for the longer-term outlook. If you want all four values in one blood draw, choose the Thyroid function Extended package. If you only want to add a single marker to the basic test, you can do so through Bloodworks Select.

Targeted testing can provide reassurance with:

• persistent fatigue
• unexplained weight change
• hair loss
• a thyroid condition in the family

A thyroid test is also worth considering if you want to follow the trend after a previously abnormal value, if you want your thyroid function checked periodically, or if you recently gave birth and have symptoms that could fit a disrupted thyroid. With questions about fertility, TSH is often included.

If you use thyroid medication, please let us know so the doctor can adjust the interpretation.

You arrange a thyroid blood draw with Bloodworks in four steps, without first needing an appointment with a doctor.

• 1Choose your test. The basic Thyroid function test measures TSH and free T4. If you also want free T3 and anti-TPO, choose Thyroid function Extended. If in doubt, the online test finder guides you in a few questions.
• 2Pay and complete the short intake. The doctor asks about your symptoms, your medication and a possible pregnancy, among other things. The doctor uses that information when assessing your result. After approval you receive your lab form digitally.
• 3Have your blood drawn at a collection point near you. Bloodworks works with a nationwide network of collection points; at most of them you can walk in without an appointment. Fasting is not needed for TSH and free T4. If you use levothyroxine, preferably have your blood drawn before you take your tablet, so your result is comparable with earlier and later measurements.
• 4Receive your report with the doctor's explanation. The laboratory analyses your blood; the result is typically available within 2 working days. A doctor of Bloodworks registered in the Dutch BIG register assesses your values against the laboratory's reference range and in the context you gave in the intake. For significantly abnormal values we contact you proactively.

Your values are kept in your personal environment, so that you can see the course over time when you repeat the test. With the thyroid in particular, that course is often more informative than a single measurement.

Your report shows each value next to the reference range of the laboratory that analysed your blood. That range is not stated in this guide, and that is deliberate: it differs per laboratory and per measuring method, and for some values also by age and stage of life. The number you find elsewhere online may therefore be slightly different from what applies to your sample. The laboratory leads, and the doctor weighs the result.

What an abnormal value does mean

• A TSH that is too high means the pituitary is pushing extra hard. With a free T4 still within range, the doctor speaks of a mild or early slowdown; if free T4 is lowered, the thyroid is clearly underactive.
• A TSH that is too low means the pituitary is easing off. With a raised free T4 the thyroid is overactive; with a free T4 within range the picture is mild.
• Raised anti-TPO makes an autoimmune cause more likely and says something about the expected course.

What an abnormal value does not mean

• A result outside the range is not a diagnosis. Reference ranges are set so that the large majority of healthy people fall within them; a small share of healthy people therefore fall outside them without anything being wrong.
• A slightly raised TSH with a free T4 within range can be temporary, for example during recovery from another illness. That is why, with a first mild abnormality, the doctor often asks for a repeat after some time before drawing conclusions.
• A result within range does not rule out every symptom. If your symptoms persist, other possible causes deserve attention.

Why the doctor weighs the context

The same numbers can mean something different depending on your situation. When assessing, the doctor therefore looks at:

• Medication: levothyroxine, antithyroid drugs, amiodarone, lithium and certain immunotherapy affect the values. Whether you took your tablet before or after the blood draw also matters.
• Pregnancy: the need for thyroid hormone rises and the values shift naturally; the usual ranges then do not simply apply.
• Illness and recovery: during and shortly after another illness, TSH and free T4 can deviate temporarily without anything being wrong with the thyroid itself. Testing is then better done later.
• Supplements: high doses of biotin (vitamin B8, often in hair and nail supplements) can interfere with the measuring method of some laboratories and give a false picture. Mention this in the intake.
• A result that does not fit the picture: if TSH and free T4 do not fit each other or your symptoms, that is a reason to repeat the measurement or check it with another method, not to draw conclusions right away.

With that context, your report gives you not a bare number but an explanation of what it means for you and what a sensible next step is.

If you already use thyroid medication

If you use levothyroxine for an underactive thyroid, your GP or specialist checks the values at fixed moments: a few weeks after each dose adjustment, then at longer intervals. Such a check belongs in that treatment pathway. A test through Bloodworks can complement it, for example to follow a trend in between, but does not replace your doctor's monitoring. Two practical points that make the result easier to compare:

• have your blood drawn at a similar time as at earlier checks, and preferably before taking your tablet
• state in the intake which medicine and which dose you use, and whether it changed recently

Self-care products such as seaweed, fish oil capsules and multivitamins with iodine can also affect your thyroid values, and so explain a result that does not fit your dose. Discuss such combinations with your GP or pharmacist.

If you use antithyroid drugs for an overactive thyroid, you are almost always followed by an internist; consult that doctor before testing on your own.

Pregnancy and wanting to conceive

In the first part of pregnancy the child uses the mother's thyroid hormone; a well-functioning thyroid is therefore important for conception, the pregnancy and the child's development. In women with a healthy thyroid, the body produces that extra amount itself; with an underactive thyroid the medication dose has to go up and the blood is checked regularly during pregnancy. The values also shift naturally during pregnancy, so the usual reference ranges do not simply apply.

If you are pregnant or want to become pregnant, always mention it in the intake; the doctor weighs it when assessing your result. After giving birth, the thyroid can also be temporarily disrupted by postpartum thyroiditis. Mention a recent birth in the intake as well.