Is your thyroid really fine? What routine thyroid testing can miss

This post goes with episode 4 of The Naturalist Podcast, "Is Your Thyroid Really Fine? Reading TSH, Free T4, Free T3 and Anti-TPO."

Most routine thyroid checks measure one marker, TSH. It's a useful first check, but it isn't a thyroid hormone, and on its own it can't show how much thyroid hormone you have, how well your body is using it, or whether your immune system is targeting your thyroid. That's why you can be told your thyroid is fine and still not feel right.

In this post I'll walk you through what your thyroid does, how it connects to your hormonal health, what each thyroid marker measures, and how to read your own results with a little more confidence.

What does your thyroid actually do?

Your thyroid is a small, butterfly-shaped gland at the base of your neck. It sets the pace for how your body uses energy, which is why it affects your temperature, digestion, mood, hair, skin and menstrual cycle.

When it's running a little slow, it can show up as:

• Tiredness that rest doesn't fix

• Foggy thinking and a poorer memory

• Poor sleep, or waking unrefreshed

• Feeling less able to cope with stress

• Low mood or anxiety-type feelings

• Cycles that get longer, heavier or more irregular

• Feeling cold when others are comfortable

• Dry skin and thinning hair

• Weight creeping up without any change in how you eat

These symptoms are common, and many of them overlap with hormonal conditions, low iron, blood sugar changes and stress. That overlap is a big part of why thyroid function gets missed.

Your thyroid works in three steps

1. The signal. Your pituitary gland, in your brain, sends out TSH (thyroid stimulating hormone). This tells your thyroid how much hormone to make.

2. The supply. Your thyroid makes mostly T4, the storage form. Think of it as raw material.

3. The conversion. Your body turns T4 into T3, the active hormone your cells actually use for energy. This happens in your liver, gut and tissues.

A routine thyroid check usually measures step one only. When your thyroid output falls, your pituitary asks for more and TSH rises. So TSH shows demand. It doesn't show how much hormone is actually there, or how much your cells can use.

Why can a normal TSH still be worth a closer look?

The TSH range on most New Zealand lab reports runs from roughly 0.3 to about 4. Anything inside that is called normal, so it isn't flagged and nobody rings you. The range I work to in clinic is 1 to 2.

So a TSH of 2.7 sits comfortably inside the lab range, but outside the range I work to. It's a number I'd want to look at more closely, especially alongside symptoms.

The upper limit for TSH has been debated by specialists for more than twenty years, and it still isn't settled. Some argue it should be lower, and in fertility medicine a cut-off of 2.5 is already used in some situations. Others argue a lower limit would label too many women with a thyroid condition. Both views are in mainstream journals.

One explanation put forward for the lower view is that the groups used to build the range can include people with thyroid antibodies they don't know about, which pushes the upper limit up.

A lab range is built to find thyroid disease. It isn't built to tell you whether your thyroid is working well for you. The range I work to isn't a diagnostic threshold either. A result outside it means it's worth a closer look, not that something is wrong. Reference ranges are set by the laboratory, so the range printed on your own report is the one that applies to your result.

What does a full thyroid panel include?

A full thyroid panel looks at four markers, read together. Most routine checks stop at the first.

TSH is the signal from your brain to your thyroid, asking it to work harder. A useful first check, but it shows demand, not supply.

• Lab range: about 0.3 to 4

• The range I work to: 1 to 2

Free T4 is the storage form your thyroid makes. It tells you whether your thyroid is producing enough raw material.

• Lab range: about 12 to 22

• The range I work to: 14 to 20

The word free matters. Most thyroid hormone in your blood is attached to a carrier protein, and only the free portion is available to your cells. Oestrogen, including oestrogen from the pill, raises that carrier protein, which is one reason the free hormones are the ones worth looking at.

Free T3 is the active hormone your cells actually use. Most of it is made by converting T4, so it depends on that conversion working well.

• Lab range: about 3.9 to 6.8

• The range I work to: 4.5 to 5.5

You can have a normal TSH and a normal T4 alongside a low free T3. That usually points to a conversion step that isn't keeping up: you're making the storage form, but not enough of the active one. It's one of the most common reasons to feel unwell when a thyroid panel looks fine.

Anti-TPO shows whether your immune system is targeting your thyroid, which is the pattern in Hashimoto's. It's the main thyroid antibody marker tested now, including in most self-requested thyroid panels.

In New Zealand, it's estimated that around one in ten women aged 30 to 65 test positive for thyroid antibodies. Most never go on to develop an underactive thyroid, so a positive result is a reason to pay attention, not a reason to panic.

Autoimmune thyroid conditions tend to develop slowly. In the early stage, anti-TPO can be raised while TSH, T4 and T3 still look normal, and that's often when fatigue and anxiety-type symptoms begin.

What else affects your thyroid results?

Your thyroid doesn't work in isolation. Converting T4 into T3 needs certain nutrients, including iron, zinc and selenium, so those results are worth reading alongside your thyroid panel. Low ferritin, your iron storage, is a common one to check.

Conversion is also affected by stress, illness and not eating enough. Under sustained stress, illness or a shortage of fuel, your body can turn more T4 into reverse T3, an inactive form. That's an adaptive response, your body's way of conserving energy, and it's one reason a single result on a single day needs context.

A few practical things can shift results on the day:

• Being unwell, fasting or under significant stress when your blood is taken

• Biotin, found in many hair, skin and nail supplements, which can interfere with some thyroid tests. If you take biotin, ask your lab or doctor how long to stop before testing.

How does your thyroid affect your hormonal health?

Your thyroid and your sex hormones are closely connected. That's why thyroid function is worth understanding if you're living with any hormonal condition, including PCOS, endometriosis, PMDD, adenomyosis or fibroids, or if your cycle and energy simply don't feel right.

Your cycle

Thyroid hormone helps regulate ovulation. When thyroid function is low, cycles can become longer, heavier or more irregular, and ovulation can become less reliable. Because progesterone is made after you ovulate, fewer ovulations can also mean lower progesterone in the second half of your cycle.

An underactive thyroid can also raise prolactin, a hormone that can interfere with ovulation when it's elevated.

Your sex hormone balance

An underactive thyroid can lower SHBG, the protein that holds testosterone in reserve. That can leave more free testosterone, bringing acne, hair changes and irregular cycles. These overlap with PCOS and can make the two hard to tell apart.

Research also suggests Hashimoto's is more common in women with PCOS. One recent meta-analysis found the odds were close to three times higher, although some studies suggest family history explains much of that.

Energy and fatigue

T3 is the hormone your cells use to make energy. When thyroid function is low, it can show up as tiredness that rest doesn't fix, a heavy afternoon slump and less stamina than you used to have. Low iron often sits alongside it, and the two can add to each other.

Brain fog and mood

Your brain is sensitive to thyroid hormone. Foggy thinking, a poorer memory, trouble concentrating, low mood and anxiety-type feelings are all associated with changes in thyroid function. Anxiety-type symptoms can also appear early in autoimmune thyroid conditions, before standard results change.

Sleep

Thyroid function and sleep influence each other. Low thyroid function is associated with poorer sleep quality and waking unrefreshed. An overactive thyroid tends to make it harder to fall and stay asleep. Ongoing poor sleep is also a stress on your body, which can affect how well you convert T4 into T3.

Stress and resilience

Sustained stress can reduce how much T4 you convert into active T3, and increase reverse T3, the inactive form. So feeling like you have less capacity to cope can be part of the same picture. Your thyroid and your stress response are closely linked.

Immune health

Hashimoto's is an autoimmune condition, where your immune system targets your thyroid. Autoimmune conditions tend to cluster, so an underactive thyroid is more common in people with other autoimmune conditions such as type 1 diabetes or rheumatoid arthritis. It also often runs in families, and autoimmune thyroid conditions are far more common in women than in men.

Fertility, pregnancy and after birth

Thyroid function matters if you're trying to conceive, which is why fertility medicine uses a TSH cut-off of 2.5 in some situations. Thyroid function can also change after giving birth, so new symptoms in the months after a baby are worth checking.

If you're living with a hormonal condition and you're exhausted, foggy, not sleeping well or your cycle has changed, it's reasonable to want your thyroid looked at properly.

What can you do with your own results?

1. Find your TSH. Pull up your last blood test and look at the number itself, not just whether it was flagged. See where it sits against the lab range and the range I work to.

2. Check what was actually run. Was it TSH alone, or the full panel of TSH, free T4, free T3 and anti-TPO? If it was TSH alone, you've seen one step out of three.

3. Consider the full panel if a close relative has a thyroid or autoimmune condition, if you're trying to conceive, or if your TSH came back normal and nothing about how you feel agrees with it.

4. Look for the reason, not just the number. A slow thyroid, a conversion problem, an immune process, low iron or zinc, and stress can all produce a similar picture. Each needs something different.

That last point is why I don't recommend thyroid supplements, including iodine and selenium, on a guess. Iodine is essential for making thyroid hormone, but too much can cause problems, particularly when there's autoimmunity. Support that suits one pattern can be wrong for another. If you take thyroid medication, any change is a conversation for your doctor.

Frequently asked questions

Is TSH a thyroid hormone?
No. TSH is made by your pituitary gland in your brain. It's the signal that tells your thyroid how much hormone to make. The thyroid hormones themselves are T4 and T3.

What is a good TSH level for women?
The lab range on most New Zealand reports runs from roughly 0.3 to about 4. The range I work to in clinic is 1 to 2. That's a practice range, not a diagnostic threshold, so a result outside it is a reason for a closer look rather than a diagnosis.

Can you have Hashimoto's with a normal TSH?
Yes. In the early stage of autoimmune thyroid disease, anti-TPO antibodies can be raised while TSH, free T4 and free T3 still look normal. That's why anti-TPO is part of a full thyroid panel.

What's the difference between free T4 and free T3?
Free T4 is the storage form your thyroid makes. Free T3 is the active form your cells use. Most free T3 comes from converting T4, so a low free T3 with a normal T4 can point to a conversion issue.

Does the pill affect thyroid results?
The pill raises the carrier protein that thyroid hormone binds to, which can shift total hormone levels. Free T4 and free T3 measure the portion actually available to your cells, which is why they're the more useful markers.

What can affect my thyroid blood test?
Being unwell, fasting or under significant stress can shift your results. Biotin, found in many hair, skin and nail supplements, can interfere with some thyroid tests, so ask your lab or doctor how long to stop it before testing.

See where your own numbers sit

The Full Picture Handbook covers every marker in the thyroid panel, with the lab range and the range I work to side by side. The Results Decoder plots your own numbers against them. Both are free, along with a free workshop on what standard testing misses.

Get the handbook and Decoder at naturalist.co.nz/testing

If you'd like support reading your results in the context of your whole history, you can book a free 15 minute Discovery Call at naturalist.co.nz.

This post is general education. It does not diagnose, and it does not replace medical care. Please do not start, stop or change any medication or supplement, including thyroid medication, based on this post without talking to your doctor or health professional.