The thyroid gland produces hormones that regulate metabolism, energy, and — critically for fertility — the hormonal environment in which ovulation, fertilisation, and early pregnancy occur. Both underactive and overactive thyroid conditions are associated with fertility problems, and subclinical thyroid dysfunction (abnormal blood tests without obvious symptoms) is common enough that it is routinely screened for in the fertility investigation panel.

Why Thyroid Function Matters for IVF

Thyroid hormones (T3 and T4) interact with the hypothalamic-pituitary-ovarian axis — the hormone signalling chain that controls ovulation. TSH (thyroid-stimulating hormone), produced by the pituitary gland, is the most reliable marker of thyroid status: high TSH suggests the thyroid is underperforming (hypothyroidism); low TSH suggests overactivity (hyperthyroidism).

Hypothyroidism and fertility:

  • Elevated TSH is associated with irregular or absent ovulation
  • Thyroid antibodies (particularly anti-TPO antibodies) are associated with reduced IVF success rates even when TSH is in the normal range
  • Untreated or suboptimally controlled hypothyroidism is associated with increased miscarriage risk
  • Overt hypothyroidism (significantly elevated TSH with symptoms) is a recognised cause of infertility

Hyperthyroidism and fertility:

  • Overactive thyroid is associated with irregular periods and reduced ovulatory frequency
  • Active hyperthyroidism during pregnancy carries significant risks for both mother and fetus
  • Treatment of hyperthyroidism before fertility treatment is important

Testing: What to Ask For

TSH: The primary screening test. Most labs use a reference range of approximately 0.4–4.0 mIU/L. However, for fertility and pregnancy purposes, many fertility specialists and NICE guidance suggest a tighter target: TSH below 2.5 mIU/L is often recommended before and during early pregnancy.

Anti-TPO antibodies (anti-thyroid peroxidase): Tests for autoimmune thyroid disease (Hashimoto's thyroiditis). Patients with elevated anti-TPO antibodies but normal TSH have an increased risk of thyroid dysfunction developing during pregnancy, and some studies show reduced IVF success rates even with normal TSH. Whether treating antibody-positive patients with normal TSH improves IVF outcomes is debated (see below).

Free T4 and Free T3: Measure the active thyroid hormones directly. Usually checked if TSH is abnormal to determine severity.

Thyroid testing is recommended by NICE as part of the standard fertility investigation. Ask your GP to include TSH in the initial fertility blood panel if it is not already planned.

TSH Targets Before and During IVF

The ideal TSH level before fertility treatment is a subject of clinical debate, but several key points are well-established:

TSH above 4.0 mIU/L before IVF: Most fertility specialists would recommend thyroid treatment (levothyroxine) to bring TSH within range before proceeding.

TSH 2.5–4.0 mIU/L: A grey zone. NICE and the British Thyroid Association note that evidence for treating this range in terms of IVF outcomes is limited, but many fertility specialists treat to a TSH target of below 2.5 mIU/L before IVF, particularly in patients who are already on levothyroxine or who are anti-TPO antibody positive. Discuss with your fertility specialist.

TSH below 2.5 mIU/L during early pregnancy is the target for women with treated hypothyroidism. Levothyroxine dose often needs increasing by approximately 25–30% from very early in pregnancy (as soon as a positive test is confirmed) — your endocrinologist or GP should advise on this in advance.

Anti-TPO Antibodies: What to Do

Patients found to have elevated anti-TPO antibodies with a normal TSH are a common scenario in fertility clinics. The evidence on whether treating this group with low-dose levothyroxine improves IVF outcomes is mixed:

  • A 2017 RCT (the TABLET trial in the UK) found that levothyroxine given to anti-TPO antibody positive women undergoing IVF did not significantly improve live birth rates
  • Some smaller studies and meta-analyses suggest a benefit, particularly in reducing miscarriage risk

Current NICE guidance (as of 2024) does not routinely recommend levothyroxine for antibody-positive women with normal TSH outside clinical trials. However, some fertility specialists and endocrinologists do prescribe it in this context, particularly for patients with a history of miscarriage.

If you have elevated anti-TPO antibodies and are preparing for IVF, asking for a referral to an endocrinologist for a specialist opinion is reasonable.

Hyperthyroidism Before IVF

Active hyperthyroidism (low TSH, high T4/T3) should be treated and stabilised before fertility treatment begins. Carbimazole and propylthiouracil (PTU) are used to treat hyperthyroidism; PTU is generally preferred in early pregnancy. Radioiodine treatment for hyperthyroidism requires a period of 6 months before attempting pregnancy and is not appropriate during treatment or pregnancy.

Stabilised, treated hyperthyroidism is compatible with IVF and pregnancy. The key is ensuring the condition is well-controlled before and during treatment.

During IVF: What Changes

For patients on levothyroxine: Thyroid function should be rechecked before starting a stimulated IVF cycle and dose adjusted if TSH is outside the target range. IVF stimulation can cause transient thyroid changes due to high oestrogen levels during stimulation.

If pregnancy is confirmed: Increase levothyroxine dose promptly (by approximately 25–30%) from the day of a positive test, pending thyroid function recheck — do not wait for the 6-week GP appointment. TSH should be rechecked at 4–6 weeks of pregnancy and at each trimester.

Frequently Asked Questions

Q: My TSH is 3.2 — is that a problem for IVF?

A: A TSH of 3.2 mIU/L is within the standard laboratory reference range but above the 2.5 target many fertility specialists prefer for IVF. Whether to treat depends on other factors — anti-TPO antibody status, history of thyroid disease, previous pregnancy losses, and individual clinic practice. Discuss with your fertility specialist whether treatment to bring TSH below 2.5 is recommended in your case.

Q: I've been diagnosed with Hashimoto's thyroiditis. Can I still have IVF?

A: Yes. Hashimoto's (autoimmune hypothyroidism, characterised by elevated anti-TPO antibodies and often rising TSH over time) is compatible with IVF. The key is ensuring TSH is in the appropriate target range before and during treatment. Patients with Hashimoto's may need levothyroxine even before TSH rises above the standard normal range.

Q: Can thyroid antibodies cause miscarriage even if my thyroid function is normal?

A: Some evidence suggests that anti-TPO antibodies are independently associated with higher miscarriage risk even with normal TSH. The mechanism is not fully understood and the evidence is contested. NICE does not currently recommend routine treatment of antibody-positive, normal-TSH patients, but the association is biologically plausible.

Q: Will IVF stimulation medication affect my thyroid?

A: High oestrogen levels during stimulation can transiently increase thyroid-binding globulin and may affect thyroid hormone measurements. Patients on levothyroxine may need dose adjustments. Your fertility clinic should check thyroid function before starting stimulation.

Q: My GP tested my TSH and said it's normal. Do I need further thyroid testing before IVF?

A: If TSH is in the normal range and there is no history of thyroid disease or symptoms, basic testing may be sufficient. However, asking for anti-TPO antibody testing is reasonable given the association with IVF outcomes and miscarriage risk in antibody-positive patients. Discuss with your GP or fertility specialist.

This article is for information only and does not constitute medical advice. Thyroid management during fertility treatment should be coordinated between your fertility specialist and GP or endocrinologist.