Question:
I have no thyroid gland left after a complete thyroidectomy, so I take thyroxine daily. Do I need to limit my dairy intake after my daily dose because it interferes with absorption?
Answer:
We often think of food and medicine as entirely separate realms – one fuels the body, the other fixes it. But they can interact in surprising ways. Grapefruit juice, for instance, can interfere with the metabolism of blood pressure-lowering and cholesterol-lowering drugs. Leafy green vegetables can interact with warfarin, a blood thinner, and dairy foods rich in calcium can bind with certain antibiotics, such as tetracycline, reducing their effectiveness.
But do dairy foods also interact with levothyroxine, the synthetic thyroid hormone taken by thousands of New Zealanders? The answer might prompt you to reconsider the timing of your morning milk and cereal or flat white.
A fully functioning thyroid gland (a butterfly-shaped gland at the front of the neck) is crucial because it plays a key role in regulating metabolism, temperature, heart rate and more. However, if your thyroid gland is not functioning correctly or it’s been surgically removed, you’re certainly not alone. Thyroid conditions are surprisingly common in New Zealand. An estimated 5% of women and 1% of men in New Zealand have a condition that affects the gland. The incidence – particularly of hypothyroidism, where the gland doesn’t produce enough hormones – tends to increase with age.
One common outcome of thyroid dysfunction is goitre, an enlargement of the thyroid gland. This swelling can be caused by iodine deficiency, autoimmune conditions such as Hashimoto’s thyroiditis, or, less commonly, thyroid nodules or cancer. In New Zealand, iodine deficiency was historically a major cause of goitre, particularly before the introduction of iodised salt in the 1920s and, more recently, the mandatory fortification of bread with iodised salt.
People who have had their thyroid removed (thyroidectomy) require lifelong thyroxine replacement. Those with hypothyroidism also need thyroxine replacement. Levothyroxine, which is a synthetic form of the thyroid hormone T4, becomes a daily essential in these cases.
However, T4 is a sensitive little molecule when it comes to absorption. The official guidance from Medsafe New Zealand is to take levothyroxine on an empty stomach with a glass of water, at least 30 minutes before breakfast.
As for whether dairy can interfere with thyroxine, the short answer is yes. A small study of 10 healthy adults, published in the journal Thyroid in 2018, found concurrent intake of cow’s milk with oral thyroid medication modestly reduced their absorption of the drug from the gut. In particular, the calcium in dairy products can bind to thyroxine in the gut, thus reducing the amount of the medication absorbed.
Given that mornings are a common time to consume dairy products such as milk, yoghurt and cheese, timing thyroid medication correctly is important. Although the 2018 study confirmed that consuming dairy with the medication reduced absorption, we don’t have any clinical trial data confirming whether, or how much, absorption is affected when dairy intake is delayed by the recommended 30-60 minutes.
A 2023 article in the Australasian Journal of General Practice recommended pregnant and postnatal women delay eating anything, including dairy products, for 30-60 minutes after thyroxine administration, and at least two hours before taking any calcium-containing supplements. These stricter recommendations are likely given because having stable thyroid hormone levels during pregnancy is crucial for the healthy development of the unborn baby.
However, the authors caution against over-applying this advice, because dairy and calcium supplements are not created equal. A two-hour wait is needed only for calcium supplements (which contain higher concentrations of calcium), not your morning milk. So, unless you’re crushing up an antacid or downing a high-dose calcium tablet, your dairy timing doesn’t need to be that strict.
As for thyroxine, a bit of patience at breakfast may go a long way towards keeping your metabolism humming. But if you’re still not feeling right, consult your GP or endocrinologist.
