Hypothyroidism (Underactive Thyroid) — NZ Condition Guide | KiwiMeds

What is Hypothyroidism?

Hypothyroidism (underactive thyroid) is a condition where the thyroid gland does not produce enough thyroid hormone. The thyroid gland, located in your neck, produces hormones that regulate your body’s metabolism — how your body uses energy. When thyroid hormone levels are too low, your body processes slow down, causing a wide range of symptoms. It is more common in women and becomes increasingly common with age. In New Zealand, it affects an estimated 2–3% of the population.

Symptoms

Hypothyroidism symptoms develop slowly and can be easy to miss or attribute to other causes. They include fatigue and sluggishness, weight gain, feeling cold (especially cold hands and feet), constipation, dry skin and hair, hair loss, slow heart rate, muscle weakness and aches, depression, poor memory and concentration, heavy or irregular menstrual periods (in women), and puffiness of the face. Symptoms vary widely — some people have many, others have few.

Causes and Risk Factors

The most common cause of hypothyroidism worldwide is Hashimoto’s thyroiditis — an autoimmune condition where the body’s immune system attacks the thyroid gland. Other causes include previous thyroid surgery or radioactive iodine treatment for hyperthyroidism, certain medications (lithium, amiodarone), iodine deficiency (rare in NZ), and radiation therapy to the neck. Risk factors include being female, family history of thyroid disease, age over 60, and having other autoimmune conditions such as type 1 diabetes or rheumatoid arthritis.

Diagnosis

Hypothyroidism is diagnosed with a blood test measuring TSH (thyroid stimulating hormone). A high TSH indicates the thyroid gland is underactive. Free T4 levels may also be measured. Testing is straightforward and widely available through GPs in New Zealand. If you have symptoms consistent with hypothyroidism, ask your GP for a thyroid function test.

Treatment in New Zealand

Hypothyroidism is treated with levothyroxine — a synthetic thyroid hormone taken as a daily tablet. Once the right dose is established (through regular blood test monitoring), most people feel completely normal. Levothyroxine is fully funded on the Pharmac schedule. The dose may need adjustment over time, particularly during pregnancy. It is usually taken on an empty stomach in the morning, at least 30 minutes before food.

NZ-Specific Information

Levothyroxine is one of the most commonly prescribed medicines in New Zealand. The funded brand is Eltroxin. Blood test monitoring (TSH) is funded through Community Health Services. Most people with hypothyroidism are managed by their GP. Specialist referral to an endocrinologist may be needed for complex cases.

Frequently Asked Questions

Will I need to take levothyroxine forever? In most cases, yes — hypothyroidism is a lifelong condition. However, some causes (such as certain medications or postpartum thyroiditis) may be temporary. Can I take other medications with levothyroxine? Some medicines and supplements affect levothyroxine absorption, including calcium, iron, and antacids — take these at least 4 hours apart. Is hypothyroidism dangerous? When untreated, it can cause serious complications, but with treatment it is very manageable.

Supplements That May Support Management

⚠️ Important: The supplements listed below have varying levels of clinical evidence. They are not a substitute for prescribed medications and should only be considered as adjunctive support under the guidance of a qualified healthcare professional. Always inform your GP or pharmacist before commencing any supplement, as interactions with prescribed medicines are possible.

• Selenium — Selenium is essential for the conversion of thyroxine (T4) to the active triiodothyronine (T3) via selenoproteins (deiodinases). In selenium-deficient Hashimoto’s thyroiditis, supplementation at 200 mcg/day has demonstrated reductions in thyroid peroxidase antibody levels and may improve thyroid function and quality of life.
• Vitamin D — Vitamin D deficiency is significantly more common in autoimmune thyroid disease (Hashimoto’s thyroiditis). Supplementation where deficiency is confirmed may reduce disease activity and improve quality of life in people with hypothyroidism.
• Magnesium — Magnesium is required for thyroid hormone synthesis and peripheral T4-to-T3 conversion. Deficiency is associated with worsening fatigue and thyroid dysfunction.
• Iron (if deficient) — Iron deficiency impairs thyroid peroxidase enzyme activity, reducing thyroid hormone synthesis. Iron deficiency anaemia is a common coexisting condition, particularly in women with hypothyroidism. Correction of iron deficiency may improve treatment response to levothyroxine.
• CAUTION: Iodine supplements — Iodine supplementation is NOT generally recommended for hypothyroidism in New Zealand where iodine intake is already marginal-to-adequate. Excess iodine can paradoxically worsen autoimmune hypothyroidism (Hashimoto’s) by triggering the Wolff-Chaikoff effect. Use only under medical supervision.

Relevant Vaccinations

Individuals living with hypothyroidism (underactive thyroid) may benefit from the following vaccinations. Please discuss your vaccination status with your GP or practice nurse, as eligibility and funding through the New Zealand National Immunisation Schedule may apply.

• Influenza (annual) — Hypothyroidism, particularly autoimmune Hashimoto’s, is associated with modestly impaired immune function. Annual influenza vaccination is recommended.
• COVID-19 — Maintaining COVID-19 vaccination is recommended. Autoimmune conditions are associated with heightened inflammatory COVID-19 responses.
• Pneumococcal — Recommended as part of standard adult preventive health, particularly for those aged 65+.

Dietary Guidance

Evidence-based dietary modifications play a meaningful role in the management of hypothyroidism (underactive thyroid). The following foods are generally recommended as part of a balanced, condition-appropriate diet. A referral to a registered dietitian may be beneficial for personalised nutritional planning.

• Adequate iodine (iodised salt, seafood, dairy) — Mild iodine deficiency is common in NZ. Using iodised salt, consuming dairy products, and eating seafood regularly provides adequate dietary iodine for thyroid hormone synthesis. Dietary sources are preferred over supplemental iodine.
• Selenium-rich foods (brazil nuts — 1–2 daily) — One to two brazil nuts per day meets the selenium RDI. Brazil nuts are the most concentrated dietary selenium source in NZ, directly supporting T4-to-T3 conversion.
• Iron-rich foods (lean meat, legumes, fortified cereals) — Ensuring adequate dietary iron supports thyroid peroxidase function and improves levothyroxine therapy effectiveness.
• Zinc-rich foods (oysters, lean meat, pumpkin seeds) — Zinc is required for thyroid hormone synthesis and receptor function.
• Cruciferous vegetables (moderate cooking) — Raw cruciferous vegetables contain glucosinolates that can reduce thyroid hormone synthesis at high intake. Cooking deactivates these compounds. Moderate consumption of cooked cruciferous vegetables is safe and nutritionally beneficial in hypothyroidism.
• Levothyroxine timing and food interactions — Levothyroxine must be taken on an empty stomach (30–60 minutes before food) as food, coffee, and calcium-rich foods significantly reduce absorption. Iron and calcium supplements must be taken 4 hours apart from levothyroxine.

Related Conditions & Medications

Related medications: Levothyroxine. Related conditions: Type 2 Diabetes.