In the present study, we found that TPOAb positivity was not associated with infertility (male or female), abortion, time to pregnancy and parity. Our study revealed there was no association between infertility and other thyroid function tests with both male and female infertility, except fot Ft4; there was a positive association between serum concentration of Ft4 among male participants with infertility.
Thyroid antibodies may affect reproductive system with alteration in TSH/T4 levels directly but other mechanisms such as association with anti thyroglobolin, abnormal innate and humoral immunity, concurrent autoimmunity (i.e, endometriosis), vitamin D deficiency can ensue implantation failure in the absence of thyroid dysfunction .
Data on the association between thyroid autoimmunity and infertility is inconclusive and limited by limited number of comprehensive population based study [13, 20,21,22]. In a cross-sectional study on 11,565 Europian origin women, they found a negative association between TPOAb levels and parity; but not with the number of pregnancies and spontaneous abortion . The difference in race and age distribution may partly explain different results.
In a case-control study in Iran by HABIB ZV et al. in 75 euthyroid women with unexplained infertility, serum anti-TPO antibodies were significantly higher in infertile euthyroid women comparing to healthy groups . In another case-control study by Gupta J et al. in 50 infertile women, anti-thyroid antibodies were more prevalent in patients with infertility  and Manhas S et al. reported in 100 infertile female, Anti-TPO Ab was independently associated with infertility irrespective of thyroid hormones levels . The positive findings of these case control studies may be due to selection biased that been mainly resulted from recruitment of cases from infertility centers. More over despite lack of normality assumption for TPOAb levels, they compared means for reporting significany. Furtheremorethe milder subjects may be spontaneously fertilized and not included in the study and this problem eliminates in a population-based study.
Our study is in agreement with study conducted by Soltanghoraee H et al.; they found no significant difference between median of TPOAb of infertile men/women and fertile ones (2.6 IU/ml vs 2.6 IU/ml in male, 2.8 IU/ml vs 2.6 IU/ml in female). Moreover they observed no association between thyroid autoimmunity and spontaneous abortion, similar to our results.
There are several studies investigate the prevalence of thyroid autoimmunity in women seeking infertility services. Marcos Abalovich et al. demonstrated no significant difference between TAI prevalence of infertile women with the control group(26.6% vs 14.5%) . K. Poppe et al. conducted a study in 438 infertile women to investigate the prevalence of thyroid antibodies in infertile women with different causes included female or male cause and idiopathic. The thyroid test and thyroid autoimmunity association were evaluated in different groups of infertile females. The prevalence of infertility was 45, 38 and 17% in the female cause, male cause and idiopathic. TPOAb positivity was higher in all women of infertile couples but with no significant difference. In subgroup infertile women only in endometriosis TPOAb positivity showed significant difference compared with controls (18% vs 8%) .
In a study by Grassi G et al. in 149 infertile couples with the evaluation of the etiology of infertility and thyroid autoantibodies (Tg Ab, microsomal Ab), 20.1% had a detectable level of TgAb and /or M Ab and in euthyroid infertile women with thyroid autoantibodies, detected no difference either in age of subjects or duration of infertility .
There is a limited number of studies reported thyroid autoimmunity among male with infertility. Thyroid autoantibodies can be associated with sperm antibodies in infertile men [26, 27]. In a study by Harald Trummer et al. that been conducted among 305 infertile men; they reported no significant correlation between thyroid dysfunction and semen parameters, except for TPOAb positivity that was positively associated with pathozoospermia (6.7% vs 1.6%, P = .036) and asthenozoospermia (7.2%vs 1.6%, P = .049) . However they have not reported the association of autoimmunity and thyroid dysfunction with male infertility despite having adequate sample size.
In the present study, we found a negative association between fT4 level in males (in both crude and adjusted models) with infertility. It has been shown that testosterone levels is decreased in hypothyroid men due to effect on hypothalamus-pituitary or alteration on prolactin secretion . Indeed there is a significant decrease in male gonadal steroids suffered from hypothyroidism . However there is no evidence to advice systematic screening for thyroid autoimmunity in infertile men without obvious clinical symptoms . In the present study, we found a fall in fT4 level in both crude and adjusted models in males was associated with infertility although the TSH level showed no significant association that insufficient sample size could be the possible cause.
The current study has several limitations. We obtain data on infertility based on a self – reported questionnaire and observing their medical records if needed. Although there is a possibility of reducing the accuracy of self – reported questionnaire, however it may not mainly influenced our results as it an acceptable method in the population-based study for infertility ; moreover history of infertility that much affects the couple life that they may never forget it and Second, we used a single measurement of thyroid test in the present study, while it should be noticed that thyroid status may be varied by the time. Even though Somwaru et al. reported that subclinical hypothyroidism can be persistent in 56% at least for a 4-year follow-up . Third, the number of men with confirmed male infertility was small and we had no adequate power for some comparison. Forth we have not assessd other parameters for assessment of thyroid autoimmunity such as anti- thyroglobulin.
The main strength of our study is its methodology; as it is one of the few population-based studies about thyroid autoimmunity on infertility, especially male infertility.
All immunoassay tests in present study were performed in the same laboratory by skilled laboratory technicians. Furthermore, we excluded those participants using thyroid medication or those with overt hypothyroidism - hyperthyroidism to eliminate the effect of treatment on TFTs.