Thyroid stimulating hormone (TSH)

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What is TSH?
Where is TSH produced?
Function of TSH
Normal TSH levels
Optimal TSH for fertility
Causes of high TSH
Causes of low TSH
TSH and fertility
When and how to test TSH
TSH in traditional Chinese medicine
Acupuncture and TSH
Chinese herbal medicine and TSH
Diet, supplements and lifestyle
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1. What is TSH?

Thyroid stimulating hormone (TSH) is a glycoprotein hormone produced by the anterior pituitary gland in response to thyrotrophin-releasing hormone (TRH) from the hypothalamus. TSH triggers the thyroid gland to produce the thyroid hormones thyroxine (T4) and triiodothyronine (T3), and to a lesser extent calcitonin. TSH is the most useful single marker of thyroid function and is the test most commonly used to screen for thyroid disease.

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 is TSH produced?

TSH is produced by thyrotroph cells in the anterior pituitary gland. The hypothalamus releases TRH into the small portal blood vessels that connect it directly to the pituitary, and TRH stimulates the pituitary to release TSH into the systemic circulation. TSH then travels to the thyroid gland in the neck, where it stimulates the production and release of thyroid hormones.

3. Function of TSH

TSH binds to TSH receptors on the cells of the thyroid gland and stimulates:

The uptake of iodine from the bloodstream into the thyroid.
The synthesis of thyroglobulin, the precursor of T4 and T3.
The release of T4 and T3 into the circulation.
The growth and proliferation of thyroid tissue.

The thyroid hormones in turn influence almost every cell in the body — regulating metabolism, body temperature, heart rate, mood, the menstrual cycle, ovulation, the thickness of the uterine lining and the maintenance of pregnancy.

4. Normal TSH levels

The standard reference range for TSH in most UK and international laboratories is approximately 0.2–4.0 mU/L. Reference ranges vary from country to country and between laboratories — some laboratories use ranges from 0.4–4.5 or even 0.5–5.0 mU/L. Always interpret your own result against the laboratory's reference range.

5. Optimal TSH for fertility

Although the standard "normal" upper limit of TSH is around 4.0 mU/L, fertility specialists generally aim for a TSH below 2.5 mU/L for women trying to conceive, and below 2.5 mU/L (or sometimes even 2.0 mU/L) during pregnancy. Some women with TSH between 2.5 and 4.0 mU/L — technically "normal" — benefit from low-dose levothyroxine to bring TSH down further, particularly if they have a history of miscarriage, anti-thyroid antibodies or otherwise unexplained infertility.

This stricter threshold reflects evidence that even subclinical thyroid dysfunction (TSH above 2.5 mU/L with normal T4) is associated with reduced fertility, increased miscarriage rates and impaired pregnancy outcomes.

6. Causes of high TSH

High TSH typically reflects an underactive thyroid (hypothyroidism). Causes include:

Hashimoto's thyroiditis — autoimmune destruction of the thyroid gland, the most common cause of hypothyroidism in iodine-replete countries.
Iodine deficiency — the most common cause worldwide; vegetarians and vegans are at higher risk.
Surgery or radioactive iodine treatment of the thyroid.
Some medications — including lithium and amiodarone.
Inadequate dose of thyroid replacement in patients already on levothyroxine.

7. Causes of low TSH

Low TSH typically reflects an overactive thyroid (hyperthyroidism). Causes include:

Graves' disease — the most common cause of hyperthyroidism.
Toxic multinodular goitre or thyroid adenoma.
Subacute thyroiditis.
Excess thyroid replacement in patients already on levothyroxine.
Pituitary disease — rare; suggests secondary hypothyroidism with a low or normal T4.

8. TSH and fertility

Even mildly raised TSH can disrupt the menstrual cycle, suppress ovulation, thin the uterine lining and increase miscarriage risk. The mechanism involves several pathways: low T3 reduces the conversion of pregnenolone to progesterone, leading to luteal phase defect; high TRH (driving the high TSH) stimulates prolactin, which suppresses GnRH; and impaired thyroid function reduces blood flow to the uterus and ovaries.

9. When and how to test TSH

TSH can be tested at any point in the menstrual cycle. It is usually measured alongside free T4, and where indicated, free T3 and thyroid antibodies (anti-TPO and anti-TG). Note that supplements such as iron, calcium and biotin can affect either the absorption of levothyroxine or the assay used to measure TSH and free T4 — these should be taken at a different time of day from levothyroxine, and biotin should be stopped for two to three days before a thyroid blood test.

Reference ranges vary from country to country. Always interpret your own result against the laboratory's reference range and discuss it with your doctor or fertility specialist.

10. TSH in traditional Chinese medicine

In traditional Chinese medicine, an underactive thyroid (high TSH) is most commonly seen as Kidney Yang deficiency, often combined with Spleen Qi and Yang deficiency. Symptoms of cold, fatigue, weight gain, fluid retention, slow bowels, slow thinking and a pale tongue with a thick coating fit this pattern very closely. An overactive thyroid (low TSH) is most commonly seen as Liver Qi stagnation transforming into Heat, with underlying Yin deficiency.

11. Acupuncture and TSH

Acupuncture has been shown in research to regulate the hypothalamic-pituitary-thyroid axis and to normalise both subclinical and overt thyroid dysfunction. By reducing stress, improving circulation and supporting Kidney Yang, acupuncture provides a useful adjunct to medical treatment of thyroid disorders, and in some cases of mild subclinical hypothyroidism it is sufficient to bring TSH back into the optimal range.

12. Chinese herbal medicine and TSH

For subclinical hypothyroidism with high TSH, Chinese herbal formulae such as You Gui Wan, Jin Gui Shen Qi Wan, Er Xian Tang and Bu Zhong Yi Qi Tang are commonly used. For hyperthyroidism with low TSH, formulae that nourish Yin and clear Heat such as Tian Wang Bu Xin Dan are used, often combined with herbs that resolve Phlegm in the neck region (Hai Zao Yu Hu Tang). Treatment is always individualised after a full TCM assessment.

13. Diet, supplements and lifestyle

To support healthy thyroid function:

Ensure adequate iodine — fish, seaweed, eggs and unrefined sea salt are good sources.
Take selenium-rich foods (Brazil nuts, fish, whole grains) — selenium is essential for converting T4 to T3.
Avoid excessive raw cruciferous vegetables (kale, cabbage, broccoli) if hypothyroid; light cooking deactivates their goitrogenic compounds.
Take iron and calcium supplements at a different time of day from levothyroxine, as they reduce its absorption.
Stop biotin supplements for two to three days before a thyroid blood test.
Reduce psychological stress.
Sleep before 10 p.m. and aim for seven to eight hours per night.
Keep the body warm and avoid prolonged exposure to cold.