There is no special diet for those taking thyroid hormone medication, with a few exceptions. However, aiming for a diet rich in fruit, vegetables and whole grains, and adopting a healthy lifestyle keeps those with thyroid disease feeling good once thyroid levels are regulated with medication.

The thyroid gland is responsible for regulating the body's metabolic rate, growth and development.

It is like the body's thermostat, said Accredited Practicing Dietician Leah Stjernqvist, as it adjusts its output of thyroid hormones from the messages received from the hypothalamus via a negative feedback loop.

"If the thyroid is under or overactive, the whole system can be thrown out of balance, becoming too fast or too slow."

Thyroid disease and associated symptoms

Hypothyroidism
When the thyroid is underactive and is not producing enough thyroid hormone, it is hypothyroidism, which causes all metabolic processes to slow down.

Hypothyroidism can result in a range of side effects, including:

• Goitre
• Fatigue
• Weight gain
• Slow heart rate
• Elevated cholesterol
• Constipation
• Sore muscles and joints
• Dry skin, brittle nails and hair
• Mood and memory changes (including depression)
• Menstrual changes
• Intolerance to cold

There are three categories of hypothyroidism

1. Hashimoto's (autoimmune, most common if iodine intake sufficient)
2. Overt hypothyroidism (symptomatic and evident in a blood test)
3. Subclinical hypothyroidism (symptoms may or may not be present, evident in blood tests).

Hyperthyroidism

If the thyroid is overactive and producing too much thyroid hormone hyperthyroidism results, and all metabolic processes become faster.

Hyperthyroidism can result in a range of side effects, including:

• Osteopenia
• weight loss
• Rapid or irregular heartbeat
• High blood pressure
• Diarrhoea
• Hand tremors
• Insulin resistance and elevated blood glucose levels
• Anxiety, irritability, sleep difficulties
• Thinning skin
• Menstrual changes
• Sweating and an intolerance to heat

Graves' Disease is the most common autoimmune hyperthyroidism which can be characterised by protruding eyeballs, dry eyes and vision difficulties.

Dietary choices can improve the health of those with thyroid disease

Though there are no particular foods or supplements that are helpful for thyroid disease, overall healthy dietary patterns can improve the health of those with thyroid disease, explained Ms Stjernqvist.

Essential nutrients such as selenium, zinc and iron are crucial to thyroid function.

"Selenium and zinc are needed for the conversion of T4 hormone (thyroxine) into the more powerful T3 (triiodothyronine), and iron deficiency has been linked to hypothyroidism, particularly Hashimoto's disease."

When iodine is lacking, the thyroid cannot produce sufficient levels of thyroid hormones, said Lite n Easy Dietician Maryl-Ann Marshall. As a result, iodine-rich foods are essential.

"It's important to eat enough iodine-rich foods to ensure healthy thyroid hormone production.

"The main dietary sources of iodine in Australia are commercially baked bread, cow's milk and fish.

"Table salt is typically fortified to include iodine; however, products such as sea salt, Himalayan rock salt and other specialty salts are not fortified with iodine and naturally container much lower levels.

"While it's important not to consume too much salt, make sure that the salt you do use is fortified with iodine – lookout for ‘iodised salt' at the supermarket," said Ms Marshall.

In Australia, it is mandatory for all baked bread, except organic bread, to be made with iodised salt.

"Organic bread is not required to be made with iodised salt, so you would need to check the label or talk to your baker.

"Dairy is also an important source of dietary iodine. If you do not consume dairy products, check the label to see whether your plant-based milk has been fortified with iodine.

"Seaweed is naturally rich in iodine and can be a good option for plant-based diets."

Iodine: a crucial mineral in thyroid hormone production

Iodine is the building block or thyroid hormones, a mineral required in trace amounts, said Ms Stjernquist.

"Iodine is found in a wide range of food sources, but the amount of iodine in foods depends on where and when the food was grown.

"Levels of iodine found in Australian soils are considered to be low.

"Fish, seafood and seaweed are the richest sources of naturally occurring iodine.

"As naturally occurring iodine is low in the Australian food supply, salt and bread are often fortified with iodine.

"Foods fortified with iodine are considered the best sources of iodine.

"However, due to the association of sodium intake and hypertension, iodised salt intake is thought to have decreased in Australia."

What to eat to meet iodine needs?

Seafood – oysters are high in iodine, but all fish and seafood will contain some iodine.

Bread (packaged) - is now made using iodised salt in Australia. This excludes organic, unpackaged and gluten-free bread.

Seaweed (kelp), dairy products and eggs – offer additional dietary sources of iodine.

Vegetables – may contain iodine; however, this depends on whether the soil they are grown in is iodine-rich.

"Adequate iodine intake should be achievable from consuming a varied healthy diet," said Ms Stjernquist.

Foods that inhibit thyroid hormone production

Foods that can interfere with iodine uptake and inhibit the production of thyroid hormone production, goitrogens, are found in brassica or cruciferous vegetables.

Cruciferous vegetables include broccoli, cauliflower, brussels sprouts, cabbage, kale, spinach, swedes, turnips, as well as soy.

"Goitrogens are more likely to impact those with already impaired thyroid function.

"However, the amount of these vegetables that you would need to ingest for this to occur is very large.

"The health benefits of consuming a diet with a variety of vegetables including brassicas for people with impaired thyroid function far outweighs the risk of goitrogens," said Ms Stjernquist.

Foods that contain goitrogens are typically healthy foods that contain a range of important nutrients, vitamins and minerals and should not be avoided, agreed Ms Marshall.

"Instead of avoiding these foods, focus on having a varied diet so that you aren't eating too many of these foods at a given time.

"You can also cook some of your veggies, by steaming, sautéing or blanching, to help reduce their goitrogen content."

It is not recommended that soy, grapefruit or calcium or iron-containing supplements are ingested within four hours of levothyroxine, a hormone replacement for those with hypothyroidism, said Ms Stjernquist.

"It should be taken on an empty stomach at least 30 minutes before food.

"Paradoxically, high levels of iodine intake can also re-induce symptoms for those with hypothyroidism.

"It is therefore advised that these clients avoid kelp supplements and seaweed snacks, as the levels of iodine in these products can vary considerably, and package labelling may not accurately reflect iodine content.

"It is also recommended that those with known hypothyroidism consult a medical professional before using supplements, such as multivitamins or prenatal supplements, due to iodine content."

Lifestyle factors for thyroid disease

If you have thyroid disease, a healthy diet, exercise, and reducing stress are important lifestyle factors for optimising health and wellbeing.

"It's important to maintain muscle mass by exercising regularly and consuming a balanced diet, with protein distributed throughout the day.

"Managing stress and getting enough sleep is also key.

"Stress interferes with the messages the thyroid gland receives from the brain, and prolonged stress can have a more significant physical manifestation for someone with thyroid disease," said Ms Stjernquist.

"It is important to eat a varied diet to ensure you consume a range of nutrients, particularly if you have impaired thyroid function and need to avoid over-consuming goitrogenic foods.

"Smoking should also be avoided, as it's an important risk factor for goitres," said Ms Marshall.