The Prevalence of Thyroid Dysfunction and Autoimmunity in Women With History of Miscarriage or Subfertility

..Thyroid function and thyroid peroxidase antibody (TPOAb) data were available for 19213 and 19237 women, respectively. The prevalence of abnormal thyroid function was 4.8% (95% CI, 4.5-5.1); euthyroidism was defined as levels of thyroid-stimulating hormone (TSH) of 0.44 to 4.50 mIU/L and free thyroxine (fT4) of 10 to 21 pmol/L. Overt hypothyroidism (TSH > 4.50 mIU/L, fT4 < 10 pmol/L) was present in 0.2% of women (95% CI, 0.1-0.3) and overt hyperthyroidism (TSH < 0.44 mIU/L, fT4 > 21 pmol/L) was present in 0.3% (95% CI, 0.2-0.3). The prevalence of subclinical hypothyroidism (SCH) using an upper TSH concentration of 4.50 mIU/L was 2.4% (95% CI, 2.1-2.6). Lowering the upper TSH to 2.50 mIU/L resulted in higher rates of SCH, 19.9% (95% CI, 19.3-20.5).

Multiple regression analyses showed increased odds of SCH (TSH > 4.50 mIU/L) with body mass index (BMI) ≥ 35.0 kg/m2 (adjusted odds ratio [aOR] 1.71; 95% CI, 1.13-2.57; P = 0.01) and Asian ethnicity (aOR 1.76; 95% CI, 1.31-2.37; P < 0.001), and increased odds of SCH (TSH ≥ 2.50 mIU/L) with subfertility (aOR 1.16; 95% CI, 1.04-1.29; P = 0.008). TPOAb positivity was prevalent in 9.5% of women (95% CI, 9.1-9.9).

Conclusions
The prevalence of undiagnosed overt thyroid disease is low. SCH and TPOAb are common, particularly in women with higher BMI or of Asian ethnicity. A TSH cutoff of 2.50 mIU/L to define SCH results in a significant proportion of women potentially requiring levothyroxine treatment.

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