Thyroid
Thyroid Health & Fertility

Thyroid
& Fertility

Subclinical hypothyroidism and thyroid autoimmunity are among the most common and most under-treated contributors to infertility and miscarriage — yet they are missed by standard laboratory reference ranges.

20%
of RPL women have thyroid antibodies
2–3×
higher miscarriage risk with anti-TPO positive
40–50%
increase in thyroid demand in first trimester
0.5–2.5
mIU/L fertility-optimal TSH target
Reference Ranges

TSH Targets for Fertility

Standard laboratory reference ranges are not optimised for fertility. The table below shows the distinction between laboratory normal and fertility-optimal targets.

Laboratory 'Normal'
0.4 – 4.0 mIU/L
Standard reference range — not optimised for fertility
Fertility-Optimal
0.5 – 2.5 mIU/L
Target for women trying to conceive or in IVF
Pregnancy Target
0.1 – 2.5 mIU/L
First trimester — thyroid demand increases 40–50%
Subclinical Hypothyroid
2.5 – 10 mIU/L
TSH elevated but T4 normal — associated with 2–3× RPL risk
Overt Hypothyroid
> 10 mIU/L
Requires levothyroxine — associated with anovulation and miscarriage
The Biology

How Thyroid Function Affects Fertility

  • →TSH receptors are expressed on granulosa cells of developing follicles. Elevated TSH directly impairs follicular development and reduces oestradiol synthesis by granulosa cells, impairing the pre-ovulatory oestrogen surge required for LH triggering.
  • →Hypothyroidism elevates prolactin via TRH (thyrotropin-releasing hormone) stimulation of pituitary lactotrophs. Hyperprolactinaemia suppresses GnRH pulsatility, reducing LH and FSH secretion and causing anovulation.
  • →Thyroid hormones regulate SHBG synthesis. Hypothyroidism reduces SHBG, increasing free oestrogen and androgen levels — contributing to oestrogen dominance and cycle irregularity.
Nutritional Support

Key Nutrients for Thyroid Function

Selenium
200 mcg/day
Selenomethionine
Reduces anti-TPO antibody titres; essential cofactor for T4→T3 conversion (deiodinase enzymes); antioxidant protection of thyroid tissue
Iodine
150–220 mcg/day
Potassium iodide or kelp (use cautiously)
Essential substrate for thyroid hormone synthesis (T4 contains 4 iodine atoms). Deficiency causes hypothyroidism; excess can worsen Hashimoto's — do not supplement beyond RDI without testing
Zinc
15–30 mg/day
Zinc bisglycinate or picolinate
Cofactor for deiodinase enzymes (T4→T3 conversion) and thyroid hormone receptor binding. Deficiency impairs both T3 production and cellular response to T3
Vitamin D
Target 100–150 nmol/L
Vitamin D3 + K2
Vitamin D receptor (VDR) is expressed in thyroid tissue. Deficiency is associated with higher anti-TPO antibody titres and increased Hashimoto's severity
Magnesium
300–400 mg/day
Magnesium glycinate
Required for TSH receptor signalling and thyroid hormone synthesis. Deficiency impairs thyroid function and is common in women with thyroid autoimmunity
Iron
Check ferritin first
Iron bisglycinate if deficient
Thyroid peroxidase (TPO) is a haem-containing enzyme — iron deficiency directly impairs thyroid hormone synthesis. Target ferritin >60 mcg/L for fertility
Thyroid & Fertility — Quick Reference
One-page PDF cheat sheet · Supplement doses, food priorities & action steps
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