By Kirstie Vesseur, Registered Clinical Nutritionist (CNA), Legacy Nutrition, NZ and Australia.
You are cold when everyone else is fine. You are tired no matter how much you sleep, your brain feels slow, and the scales are creeping up even though you have not changed a thing. You ask your GP whether it could be your thyroid, and you are told your results are normal.
I see this often in clinic. Sometimes the thyroid really is fine and something else is going on. Sometimes it is not working the way it should and a single test has not shown the whole picture. Here is how I think it through.
What your thyroid does
Your thyroid is a small gland in your neck that sets the pace for your metabolism. Its hormones influence:
energy and body temperature
heart rate
digestion and bowel regularity
mood and thinking
hair, skin and nails
your menstrual cycle and fertility
The thyroid mainly makes T4, a storage form. Most of your active hormone, T3, is made by converting T4 in tissues such as the liver, kidneys and muscle. TSH is the signal from your brain that tells the thyroid how hard to work.
Signs your thyroid may not be working well
fatigue and low stamina
feeling cold all the time
weight gain or difficulty losing weight
constipation
dry skin, thinning hair or hair falling out
low mood and brain fog
heavier or irregular periods
None of these is specific to the thyroid, which is exactly why testing matters.
Why is my thyroid not working? Common causes
Hashimoto's
In countries with enough iodine, including New Zealand and Australia, Hashimoto's thyroiditis is the most common cause of an underactive thyroid. It is an autoimmune condition where the immune system gradually damages the thyroid. Thyroid antibodies can be raised for years before TSH moves out of range.
Slower conversion of T4 to T3
Because T3 is the active hormone, conversion matters. The enzymes that do it contain selenium, and iron and zinc also play a part. T3 tends to fall with:
significant illness or inflammation
under-eating or long periods of strict dieting
very high cortisol, for example during major stress or illness
In a two-year calorie restriction trial, T3 dropped by around 20%, although it stayed within the normal range. Reverse T3, an inactive form, can rise during stress or illness, but it is not routinely tested and its meaning in everyday practice is still debated.
Iodine: too little or too much
Iodine is needed to make thyroid hormones. New Zealand soils are low in iodine, which is why most commercial bread has been made with iodised salt since 2009 (organic and unleavened breads are exempt). Women who eat little bread, seafood, eggs or dairy can run low. On the other hand, kelp and seaweed tablets contain highly variable amounts of iodine and can go well over the safe upper limit, which is unhelpful in Hashimoto's.
Low iron and vitamin D
The enzyme that makes thyroid hormone, thyroid peroxidase, depends on iron, so iron deficiency can impair hormone production. My article on low ferritin explains why women run low. Low vitamin D is also more common in women with Hashimoto's, although this link does not prove that one causes the other.
Life stages
Thyroid problems are more common in women and become more common with age. The year after birth is another time to watch, as postpartum thyroiditis can cause a period of over- then underactivity.
What a standard test may not show
Most New Zealand labs test TSH first and add free T4 or free T3 only when the TSH result or clinical details call for it. TSH is a good screen, but on its own it does not show antibodies. If you have ongoing symptoms, it is reasonable to ask your GP about:
TSH and free T4
thyroid peroxidase (TPO) antibodies, to check for Hashimoto's
free T3, where your GP feels it is clinically relevant
ferritin and iron studies, vitamin D and B12
Reference ranges are wide. Where your result sits, along with your symptoms, antibodies and life stage, all matter. If you are planning a pregnancy, your GP may aim for a tighter TSH target. My post on thyroid health for women goes deeper into testing, and the free Check Your Bloods page helps you plan what to ask for.
If you already take thyroid medication, please do not change your dose yourself. Any change should be made with your GP.
How nutrition can help
Selenium
A 2024 meta-analysis of 35 trials found that selenium lowered TPO antibodies in Hashimoto's, with side effects no different from placebo. It did not clearly change TSH or free T4 in people already on medication, and the certainty of evidence for the antibody result was low, so think of selenium as support rather than treatment.
Food first: one Brazil nut a day provides roughly 70 to 90 mcg of selenium, although content varies with the soil. Eggs, seafood, chicken and mushrooms add more.
If you supplement: trials mostly used 100 to 200 mcg a day, often as selenomethionine, for three to six months, then a review with your practitioner.
Do not double up: choose Brazil nuts or a supplement, not both. The upper limit for adults in New Zealand and Australia is 400 mcg a day from all sources, and too much selenium is harmful.
Iodine from food
Adult women need about 150 mcg of iodine a day. Most women can meet this with regular bread, iodised salt at home, two to three serves of dairy, eggs, and fish or seafood twice a week. Avoid kelp or seaweed supplements unless your GP advises them. If you are pregnant or breastfeeding, Te Whatu Ora recommends a 150 mcg iodine-only tablet daily. If you have a thyroid condition, check this with your GP or midwife first.
Zinc and iron
Zinc: oysters, red meat, pumpkin seeds and cashews.
Iron: red meat, mussels and lentils, with a vitamin C food such as capsicum or kiwifruit alongside.
Please test before taking iron supplements, as too much iron is harmful.
Eat enough
Under-eating lowers T3. For most women with thyroid symptoms, regular meals with a palm of protein, a fist of slow carbohydrates such as kumara or oats, and plenty of vegetables work better than strict dieting.
Cooked cruciferous vegetables are fine
Broccoli, cabbage and cauliflower are only a concern in very large raw amounts, especially when iodine is low. Lightly cooked, they are a valuable part of your diet.
If you take levothyroxine
Take it on an empty stomach, 30 to 60 minutes before food, coffee or other medicines. Keep iron and calcium supplements at least four hours away from your dose. Ask your GP or pharmacist how best to fit any supplements around your medication.
Check with your GP or pharmacist before starting any supplement if you take medication, are pregnant or breastfeeding.
Lifestyle factors
Sleep: TSH naturally peaks overnight, so a regular sleep rhythm supports normal hormone patterns.
Stress: very high cortisol can lower TSH output, so daily nervous system support is worth building in.
Gut health: coeliac disease is more common in people with autoimmune thyroid disease, so it is worth being tested if you have gut symptoms, before cutting out gluten.
Thyroid symptoms overlap with many others. If tiredness and tingling are part of your picture, my post on low B12 is worth a read too.
Frequently asked questions
Can my thyroid be the problem if my TSH is normal?
Sometimes. A normal TSH makes an underactive thyroid much less likely, but it does not show antibodies. If symptoms persist, a fuller panel can add useful information. It is just as important to look at other causes, such as iron, B12 and sleep.
Should I avoid gluten if I have Hashimoto's?
Get tested for coeliac disease first, while you are still eating gluten. Some women without coeliac disease feel better reducing gluten, but this is best trialled in a structured way.
Can nutrition replace thyroid medication?
No. If you need thyroid medication, nutrition works alongside it, not instead of it.
Where to start
If you are living with thyroid symptoms and feel like you are only seeing part of the picture, start with a full set of bloods and a steady, nourishing way of eating. If you want recipes that make this easier, Legacy's Kitchen has high-protein wholefood meals ready to go.
Want help with this one-on-one? See how I work with women on women’s hormone support, or browse all my services.
This article is general education, not personal medical advice. Please talk with your GP before starting supplements or changing medication.
References
Huwiler VV, et al. Selenium supplementation in patients with Hashimoto thyroiditis: a systematic review and meta-analysis of randomized clinical trials. Thyroid. 2024. https://pmc.ncbi.nlm.nih.gov/articles/PMC10951571/
Te Whatu Ora Health New Zealand. Iodine. Accessed 2026. https://www.tewhatuora.govt.nz/for-the-health-sector/health-sector-guidance/iodine
NHMRC. Nutrient Reference Values for Australia and New Zealand: Selenium. Accessed 2026. https://www.eatforhealth.gov.au/nutrient-reference-values/nutrients/selenium
Healthify He Puna Waiora. Levothyroxine. Accessed 2026. https://healthify.nz/medicines/l/levothyroxine/
Zimmermann MB, Köhrle J. The impact of iron and selenium deficiencies on iodine and thyroid metabolism. Thyroid. 2002. https://pubmed.ncbi.nlm.nih.gov/12487769/
Roy A, et al. Prevalence of celiac disease in patients with autoimmune thyroid disease: a meta-analysis. Thyroid. 2016. https://pubmed.ncbi.nlm.nih.gov/27256300/
Ravussin E, et al. A 2-year randomized controlled trial of human caloric restriction (CALERIE). J Gerontol A Biol Sci Med Sci. 2015. https://pubmed.ncbi.nlm.nih.gov/26187233/
Wang J, et al. Meta-analysis of the association between vitamin D and autoimmune thyroid disease. Nutrients. 2015. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4425156/
