What is Thyroid Function Tests?
💡 Thyroid function tests = TSH + free T4 + free T3 + TPO antibodies. TSH is the primary screen. Preconception target: TSH 0.5–2.5 mIU/L. Hypothyroidism (TSH >4.5) = anovulation, miscarriage — treat with levothyroxine. Hyperthyroidism (TSH <0.1) = ovulatory dysfunction. Positive TPO antibodies + normal TSH still warrants treatment before IVF.
Thyroid function tests (TFTs) are a panel of blood tests assessing the activity of the thyroid gland and pituitary-thyroid axis. The standard fertility panel includes TSH (primary screen), free T4 (fT4), free T3 (fT3), and thyroid peroxidase (TPO) antibodies. Thyroid disorders are among the most common and most correctable causes of anovulation, implantation failure, and miscarriage in women.
🇮🇳 India Context: Thyroid Function Tests is widely assessed and treated across major Indian fertility centres including Chennai, Mumbai, Bangalore, Delhi, and Hyderabad.
What are the key characteristics of Thyroid Function Tests?
- TSH (thyroid-stimulating hormone): pituitary hormone controlling thyroid; the primary screening test; elevated = hypothyroidism, suppressed = hyperthyroidism
- Free T4 (fT4): active thyroid hormone secreted by the thyroid; normal 12–22 pmol/L; low fT4 + high TSH = overt hypothyroidism; normal fT4 + high TSH = subclinical hypothyroidism
- Free T3 (fT3): most metabolically active thyroid hormone; T4 is peripherally converted to T3; measured when hyperthyroidism is suspected
- TPO antibodies (anti-thyroid peroxidase): marker of Hashimoto's thyroiditis (autoimmune hypothyroidism); associated with miscarriage even when TSH is normal (2.0–4.0 mIU/L)
- Anti-TG antibodies (anti-thyroglobulin): second autoimmune marker; less specific than TPO; measured in Hashimoto's workup
- Subclinical hypothyroidism (SCH): TSH 2.5–4.5 mIU/L + normal fT4 — clinically controversial but treated in fertility/IVF context due to miscarriage risk
- Thyroid stimulating immunoglobulins (TSI): measured in Graves' disease (autoimmune hyperthyroidism); can cross placenta and affect fetus
- Thyroid USS: ordered if TPO antibodies positive, thyroid enlarged (goitre), or nodule suspected; does not replace blood tests
How does Thyroid Function Tests work?
Why does Thyroid Function Tests matter in fertility?
Thyroid function testing is the highest-yield hormonal screen after FSH/AMH/prolactin in the female fertility workup. Untreated hypothyroidism causes: anovulation (TSH >4.5), luteal phase deficiency, poor implantation, and first-trimester miscarriage. Positive TPO antibodies increase miscarriage risk by 2–3 fold even when TSH appears normal by standard references — this is why the fertility reference range (<2.5 mIU/L) rather than the laboratory reference range (0.4–4.5 mIU/L) must be used in TTC women. Every IVF programme should screen TSH before cycle start — IVF stimulation raises estrogen, which increases thyroid-binding globulin and T4 demand, potentially unmasking subclinical hypothyroidism mid-cycle.
What are related terms to Thyroid Function Tests?
TSH (Thyroid-Stimulating Hormone)
TSH (thyroid-stimulating hormone) is secreted by the pituitary gland and regulat…
Prolactin
Prolactin is a pituitary hormone primarily responsible for milk production after…
Recurrent Miscarriage
Recurrent Miscarriage means two or more pregnancy losses before 20 weeks. It aff…
Ovulation
Ovulation is the release of a mature oocyte (egg) from a dominant ovarian follic…
Preconception Care
Preconception care refers to the medical evaluation, health optimisation, and li…
FAQs about Thyroid Function Tests
What thyroid tests should I have before fertility treatment?
Minimum thyroid panel for fertility: TSH + TPO antibodies. Add free T4 if TSH is abnormal (>2.5 or <0.1 mIU/L). Preconception TSH target: 0.5–2.5 mIU/L. Positive TPO antibodies + TSH >2.0 mIU/L = start low-dose levothyroxine even if TSH appears "normal" by lab reference. Free T3 is added only if hyperthyroidism is suspected (suppressed TSH). Anti-TG antibodies added if Hashimoto's evaluation is incomplete.
What is subclinical hypothyroidism and does it affect fertility?
Subclinical hypothyroidism (SCH) = TSH elevated (2.5–4.5 mIU/L) with normal free T4. It is asymptomatic or mildly symptomatic. In the general population, treatment is controversial. In fertility medicine, SCH is treated because it: (1) is associated with 2x higher miscarriage risk; (2) impairs implantation; (3) worsens during IVF stimulation. Low-dose levothyroxine (25–50 mcg) to achieve TSH <2.5 mIU/L is standard at fertility centres.
What are TPO antibodies and why do they matter in fertility?
TPO (thyroid peroxidase) antibodies are the immune markers of Hashimoto's thyroiditis — the commonest cause of hypothyroidism. Positive TPO antibodies are associated with a 2–3-fold higher miscarriage risk and lower IVF success rates, even when TSH is normal. Meta-analyses show that treating TPO-positive women with levothyroxine (targeting TSH <2.5 mIU/L) significantly reduces miscarriage risk. Selenium supplementation (200 mcg/day) reduces TPO antibody titres and is safe during preconception.
Can hyperthyroidism affect fertility?
Yes. Hyperthyroidism (TSH <0.1 mIU/L, elevated fT4/fT3) causes: irregular periods, anovulation, increased miscarriage rate, low birth weight, premature birth. Graves' disease (autoimmune hyperthyroidism) is the most common cause. Treatment: carbimazole or propylthiouracil (PTU — preferred in first trimester if treatment needed during pregnancy). Radioiodine ablation: avoid for 6+ months before conception. IVF should not be undertaken while hyperthyroid — treat and stabilise first.
How quickly does levothyroxine work for fertility?
TSH responds to levothyroxine within 4–6 weeks of dose initiation. Recheck TSH at 6 weeks; titrate dose if needed; recheck again in 4 weeks. Target: TSH 0.5–2.5 mIU/L. Once target is achieved, IVF cycle can start. For naturally conceiving couples, regular TSH monitoring continues — TSH should be rechecked at 6 weeks pregnant (levothyroxine dose typically increased by 25–30% immediately on positive pregnancy test, before the first antenatal appointment).
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