What is TSH?
💡 TSH controls thyroid function. Optimal preconception TSH: 0.5–2.5 mIU/L. Elevated TSH (>2.5 mIU/L in TTC women): associated with anovulation, implantation failure, miscarriage. TSH >4–5 mIU/L = overt hypothyroidism — treat with levothyroxine. Subclinical hypothyroidism (2.5–4.5 mIU/L) + positive TPO antibodies = treat before IVF.
TSH (thyroid-stimulating hormone) is a pituitary hormone that regulates thyroid gland function. In fertility medicine, TSH testing is essential because both hypothyroidism (elevated TSH) and hyperthyroidism (suppressed TSH) impair ovulation, affect egg quality, and significantly increase miscarriage risk. The optimal TSH range for fertility and pregnancy differs from the general population reference range.
🇮🇳 India Context: TSH is widely assessed and treated across major Indian fertility centres including Chennai, Mumbai, Bangalore, Delhi, and Hyderabad.
What are the key characteristics of TSH?
- TSH reference range (general population): 0.4–4.5 mIU/L — but this is NOT the optimal range for conception or pregnancy
- Preconception/fertility optimal TSH: 0.5–2.5 mIU/L (ATA, ASRM guidelines); reduces miscarriage risk vs TSH 2.5–4.5 mIU/L
- Subclinical hypothyroidism (SCH): TSH 2.5–4.5 mIU/L with normal free T4 — may impair implantation, embryo quality, and increase miscarriage risk
- Overt hypothyroidism: TSH >4.5 mIU/L + low free T4 — causes anovulation, irregular cycles, infertility, and high miscarriage risk if untreated
- Hyperthyroidism: TSH <0.1 mIU/L — causes ovulatory dysfunction, low birth weight; treat before IVF
- Thyroid peroxidase (TPO) antibodies: positive in Hashimoto's thyroiditis; associated with higher miscarriage risk even when TSH is normal; indication to treat with levothyroxine before IVF
- Levothyroxine therapy: targets TSH <2.5 mIU/L preconception; increased by 25–30% on confirmed pregnancy to meet increased gestational demand
- TSH in IVF: gonadotropin stimulation raises estradiol, which increases thyroid-binding globulin → T4 demand increases → SCH may worsen during stimulation; check TSH before each IVF cycle
How does TSH work?
Why does TSH matter in fertility?
Thyroid dysfunction is the second most common endocrine cause of female infertility after PCOS. Its correction is one of the highest-yield interventions in fertility medicine — a woman with a TSH of 4.0 mIU/L and positive TPO antibodies who starts levothyroxine may achieve a natural pregnancy within 3–6 months without IVF. The key clinical error is using the general population reference range (0.4–4.5) rather than the preconception target (<2.5 mIU/L) — this results in under-treatment of subclinical hypothyroidism in women trying to conceive. Every fertility workup should include TSH as a minimum; TPO antibodies should be added when TSH is borderline or the patient has a family history of thyroid disease.
What are related terms to TSH?
Thyroid Function Tests
Thyroid function tests are a panel of blood tests assessing thyroid health — pri…
Prolactin
Prolactin is a pituitary hormone primarily responsible for milk production after…
Ovulation
Ovulation is the release of a mature oocyte (egg) from a dominant ovarian follic…
Recurrent Miscarriage
Recurrent Miscarriage means two or more pregnancy losses before 20 weeks. It aff…
Preconception Care
Preconception care refers to the medical evaluation, health optimisation, and li…
FAQs about TSH
What is the normal TSH level for fertility?
The general population normal TSH range is 0.4–4.5 mIU/L — but the preconception fertility target is 0.5–2.5 mIU/L, as recommended by the American Thyroid Association (ATA) and ASRM. Women with TSH 2.5–4.5 mIU/L who are trying to conceive or undergoing IVF should be offered low-dose levothyroxine (25–50 mcg), especially if TPO antibodies are also positive. TSH >4.5 mIU/L = overt hypothyroidism — always treat before IVF.
Can high TSH prevent pregnancy?
Yes. Elevated TSH (hypothyroidism) suppresses ovarian function, reduces FSH/LH pulsatility, impairs follicle development, and significantly increases miscarriage risk. TSH >4.5 mIU/L causes anovulation and irregular cycles in many women. Subclinical hypothyroidism (TSH 2.5–4.5 mIU/L) is associated with implantation failure and first-trimester miscarriage. Treating with levothyroxine to achieve TSH <2.5 mIU/L can restore ovulation and significantly reduce miscarriage risk.
Does TSH need to be checked before IVF?
Yes — mandatory. TSH should be measured before every IVF cycle. IVF stimulation with gonadotropins raises estradiol, which increases thyroid-binding globulin (TBG) and T4 demand — potentially unmasking or worsening subclinical hypothyroidism mid-cycle. Many IVF protocols require TSH <2.5 mIU/L as a prerequisite for cycle start. If TSH is elevated, treatment with levothyroxine is initiated and IVF delayed 6 weeks until the target is achieved.
What is the relationship between TSH and miscarriage?
TSH is one of the most important modifiable miscarriage risk factors. Studies show: TSH >2.5 mIU/L in the first trimester is associated with 2–3x higher miscarriage rate. Positive TPO antibodies (Hashimoto's thyroiditis) increase miscarriage risk even when TSH is in the normal range (1–4 mIU/L). Treating women with TSH 2.5–4.5 mIU/L and positive TPO antibodies with low-dose levothyroxine reduces miscarriage risk in multiple RCTs.
Should I take levothyroxine if my TSH is 3.5 mIU/L and I am trying to conceive?
The decision depends on context. If TSH 3.5 mIU/L + positive TPO antibodies: most fertility specialists recommend low-dose levothyroxine (25 mcg) to reduce miscarriage risk — target TSH <2.5 mIU/L. If TSH 3.5 mIU/L + TPO antibodies negative + no prior miscarriages: evidence is less clear; some centres treat prophylactically for IVF. Discuss with your fertility specialist — this is one of the most impactful, lowest-risk interventions available if the indication is present.
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