Thyroxine (T4) and triiodothyronine (T3)

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What are T4 and T3?
Where are T4 and T3 produced?
Function of T4 and T3
T4, T3 and fertility
Normal T4 and T3 levels
Causes of high T4 and T3
Causes of low T4 and T3
When and how to test T4 and T3
T4 and T3 in traditional Chinese medicine
Acupuncture and thyroid hormones
Chinese herbal medicine and thyroid hormones
Diet, supplements and lifestyle
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1. What are T4 and T3?

Thyroxine (T4) and triiodothyronine (T3) are the two main thyroid hormones. T4 is the major form produced by the thyroid gland — it is relatively inactive, but is converted to the much more active T3 in peripheral tissues. T3 is the form that actually binds to thyroid hormone receptors throughout the body and drives metabolic and reproductive activity.

Reference ranges vary from country to country and between laboratories. Always interpret your own result against the reference range provided by the laboratory that performed your test.

2. Where are T4 and T3 produced?

T4 and T3 are produced by the thyroid gland in response to TSH from the pituitary. The thyroid releases approximately 80% T4 and 20% T3 directly into the circulation. Additional T3 is produced in peripheral tissues — particularly the liver, kidneys and skeletal muscle — by the conversion of T4 to T3 by deiodinase enzymes. Selenium is essential for these enzymes to function.

3. Function of T4 and T3

Thyroid hormones influence almost every cell in the body. Their main actions are:

Regulation of basal metabolic rate.
Maintenance of body temperature.
Regulation of heart rate and cardiac output.
Support of normal brain function and mood.
Regulation of growth and development in children.
Support of normal menstrual cyclicity, ovulation, the uterine lining, libido and fertility.
Support of normal fetal development during pregnancy — particularly brain development in the first trimester.

4. T4, T3 and fertility

Thyroid hormones, particularly T3, have a profound effect on fertility:

T3 enhances the conversion of pregnenolone to progesterone, supporting the luteal phase and early pregnancy.
Adequate thyroid hormone levels are needed for normal ovulation and a healthy uterine lining.
Thyroid dysfunction is one of the most easily treated causes of unexplained infertility, miscarriage and luteal phase defect.
In men, normal thyroid function is required for healthy sperm production and motility.

Women with subclinical hypothyroidism (raised TSH with normal T4) often have low-normal T3 in the luteal phase, which contributes to inadequate progesterone and luteal phase defect.

5. Normal T4 and T3 levels

Typical reference ranges are approximately:

Free T4: 9–25 pmol/L (or 0.7–1.9 ng/dL).
Free T3: 3.1–6.8 pmol/L (or 2.0–4.4 pg/mL).

Reference ranges vary from country to country and between laboratories. Always interpret your own result against the laboratory's reference range. For optimal fertility, free T3 should be in the upper half of the reference range and free T4 in the upper third.

6. Causes of high T4 and T3

High T4 and T3 typically reflect hyperthyroidism. Causes include Graves' disease, toxic multinodular goitre, thyroid adenoma, subacute thyroiditis, and excessive thyroid hormone replacement.

7. Causes of low T4 and T3

Low T4 and T3 typically reflect hypothyroidism — most commonly Hashimoto's thyroiditis, iodine deficiency, or surgery/radioactive iodine treatment of the thyroid. Selenium deficiency can also impair T4-to-T3 conversion, producing a pattern of normal T4 with low T3.

8. When and how to test T4 and T3

Free T4 and free T3 are usually measured alongside TSH as part of a full thyroid panel. They can be tested at any point in the menstrual cycle. Some women have a normal TSH and free T4 but a low free T3 — this "low T3 syndrome" is missed by TSH-only testing and is an important cause of fertility problems. Anti-TPO and anti-TG antibodies should also be measured to identify autoimmune thyroid disease, which can affect fertility even when TSH is normal.

Reference ranges vary from country to country. Always interpret your own result against the laboratory's reference range.

9. T4 and T3 in traditional Chinese medicine

In traditional Chinese medicine, low T4 and T3 (hypothyroidism) are most commonly seen as Kidney Yang deficiency, often combined with Spleen Yang deficiency. Symptoms of cold, fatigue, weight gain, fluid retention and slow bowels fit this pattern. High T4 and T3 (hyperthyroidism) are seen as Liver Qi stagnation transforming into Heat, with underlying Yin deficiency — symptoms of palpitations, irritability, sweating, weight loss and trembling.

10. Acupuncture and thyroid hormones

Acupuncture has been shown in research to regulate the hypothalamic-pituitary-thyroid axis and to support healthy T4 and T3 levels. In subclinical hypothyroidism, acupuncture is a useful adjunct to medical treatment, and in some cases is sufficient to bring TSH and thyroid hormones into the optimal range. Acupuncture also helps reduce thyroid antibodies in some patients with autoimmune thyroid disease.

11. Chinese herbal medicine and thyroid hormones

For low thyroid hormones (high TSH), Chinese herbal formulae such as You Gui Wan, Jin Gui Shen Qi Wan and Er Xian Tang are commonly used. For high thyroid hormones (low TSH), Tian Wang Bu Xin Dan and Hai Zao Yu Hu Tang are used. Treatment is always individualised.

12. Diet, supplements and lifestyle

To support healthy thyroid hormone production and conversion:

Ensure adequate iodine intake — fish, seaweed, eggs, unrefined sea salt.
Take selenium-rich foods — Brazil nuts (just 2–3 per day provides the daily requirement), fish, whole grains.
Ensure adequate zinc and iron intake — both are required for thyroid hormone production.
Avoid excessive raw cruciferous vegetables in hypothyroidism.
Reduce psychological stress.
Avoid prolonged dieting and undereating, both of which suppress T3 production.
Sleep before 10 p.m. and aim for seven to eight hours per night.