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Thyroid & Fertility

Thyroid disorders are easily overlooked, but they’re crucial fertility conditions, especially hypothyroidism (an underactive thyroid) which is more common than hyperthyroidism. It reduces male libido and sperm count, but has even more significant effects on female fertility and increases the chances of the following conditions:

  1. Abnormal menstrual cycle patterns
  2. Endometriosis
  3. Oligomenorrhea or amenorrhea i
  4. Lower conception rates, both naturally and with ART
  5. Higher risks of complications in pregnancy and for the child
  6. The diagnosis of “unexplained infertility

Having an underactive thyroid is a surprisingly common issue:

  • 13.9% of women diagnosed with infertility have subclinical hypothyroidism ii
  • 4–10% of the general population have subclinical hypothyroidism
  • 7-26% of the elderly population have subclinical hypothyroidism
  • Overt hypothyroidism in the general population is much lower at around 0.3% iii

Interlinking hormone systems

The hormonal (endocrine) system is complex, and the hypothalamus and pituitary glands are both part of the thyroid, adrenal and reproduction systems, and problems in one axis can affect the others: iv

  1. The hypothalamic-pituitary-thyroid axis (HPT)
  2. The hypothalamic-pituitary-adrenal axis (HPA)
  3. The hypothalamic-pituitary-ovarian axis (HPO)

The main connections between thyroid function and fertility are:

  1. Thyroid hormone receptors on the ovaries alter how they function
  2. Estrogens from the ovaries affect the hypothalamus, which controls both the HPT and HPO axis
  3. When the hypothalamus produces high levels of thyroid releasing hormone (TRH) to encourage the pituitary to make more thyroid-stimulating hormone (TSH), the pituitary also produces prolactin”, and too much prolactin stops menstrual cycles (hyperprolactinaemia)
  4. Low T3 and T4 levels affect reduce sex-hormone-binding globulin (SHBG) levels and increase prolactin and TSH from the anterior pituitary. The outcome of this is lower FSH and LH levels which are essential for menstrual cycles

Most cases of hypothyroidism are subclinical, and low thyroid hormone levels are a real concern when trying to conceive or sustain a pregnancy as the body needs more thyroid hormones (especially weeks 6 to 20), and they must stay relatively high for months after the birth.

Hypothyroidism and ART

Hormonal stimulation of women with low thyroid levels in assisted reproductive techniques (ART) increases the potential complications for them. ACEE recommend all women considering ART should have their thyroid function checked because:

  • Controlled ovarian hyperstimulation (COH) substantially increases circulating estrogen concentrations which can severely impair thyroid function vii
  • For women without thyroid antibodies, these changes pass, but if they have thyroid autoimmunity, the estrogen stimulation can cause abnormal thyroid function for the rest of the pregnancy
  • Thyroid autoimmunity is significantly higher among infertile women than among fertile women, especially when women have endometriosis or premature ovarian failure viii

Diagnosing subclinical hypothyroidism

Diagnosing is quite difficult as symptoms may be mild or absent. As the body’s demand for thyroid hormones increases in pregnancy, a previously unnoticed thyroid disorder may worsen and become more serious. Unfortunately, symptoms in pregnancy aren’t always typical and may be indistinguishable from the signs of a normal pregnancy. A raised awareness is essential, especially for women with a higher risk of thyroid disease, who have:

  • A personal or family history of thyroid disease
  • A goitre (swelling of the neck)
  • An autoimmune disorder such as Type 1 diabetes
  • High levels of stress
  • Had treatment for an overactive thyroid gland

Treatments for hypothyroidism

  • Treatment must increase the levels of thyroxine (T4) to a healthy level and reduce the level of TSH or enable the transformation of T4 to T3.
  • A reduction in TRH production from the hypothalamus allows TSH and prolactin levels to return to normal, and the pituitary then releases enough follicle-stimulating hormone (FSH) to trigger ovulation.
  • Fertility, as well as energy levels, rise, and ART can often be avoided.
    1. L-thyroxine (T4) supplements are needed to treat overt hypothyroidism. The dose is generally lower in patients with subclinical hypothyroidism than in the overt form. Average daily doses vary from 25 to 75μg (depend­ing on the TSH elevation), and adjustments are often needed to reach a stable level.
    2. Supplements of iodine (and other minerals) are needed for thyroxine (T4) production and its conversion to T3. The conversion of T4 to T3 happens in the liver and muscles, and it requires zinc, copper, manganese and selenium (and T2) to complete the process. Supplementing often works well for subclinical hypothyroidism and is a viable alternative approach that can avoid lifelong thyroxine supplementation. However, poor conversion rates of T4 to the far more active T3 only show up in tests that assess both hormones. However, iodine supplements during pregnancy is not universally recommended as it can cause thyroid issues for the child. Low iodine in pregnancy and postpartum is associated with lower TSH and higher T3 and T4 levels. Iodine-containing supplements taken before and continued through pregnancy are associated with lower TSH and higher T3 and T4 concentrations, which may suggest improved thyroid function. ix The American Thyroid Association, the Endocrine Society, and the American Academy of Pediatrics recommend that all pregnant and lactating women take a daily multivitamin or mineral supplement containing 150 μg of iodine. x
    3. The diet is vital as all thyroid hormones are derived from an amino acid called L-tyrosine, present in high protein foods such as meats, peanuts, seeds and dairy products. A diet without these can cause low thyroid hormone levels and is typically an issue for vegans who usually need to supplement L-tyrosine to regain thyroid function.
    4. Low thyroid levels typically generate a Cold PFP, which can affect the fertility of both sexes. We recommend following personal fertility profile advice, as well as taking specific action needed to address abnormal thyroid function.

Thyroid UK has excellent advice and research on the thyroid.

i Hypothyroidism among infertile women in Finland ‘.M. Arojoki et al. Gynecological Endocrinology 2000, Vol. 14, No. 2 , Pages 127-131. (doi/abs/10.3109/09513590009167671)
iiSubclinical hypothyroidism and thyroid autoimmunity in women with infertility ‘. Marcos Abalovich et al. Gynecological Endocrinology 2007, Vol. 23, No. 5 , Pages 279-283 (doi:10.1080/09513590701259542)
iii “Serum TSH, T(4), and thyroid antibodies in the United States population (1988 to 1994): Hollowell JG, Staehling NW, Flanders WD, et al. (February 2002). National Health and Nutrition Examination Survey (NHANES III)”. J. Clin. Endocrinol. Metab. 87 (2): 489–99. PMID 11836274
iv The hypothalamic-pituitary-thyroid axis and the female reproductive system. ‘ Doufas AG, Mastorakos G. Ann N Y AcadSci. 2000;900:65-76.
v Subclinical Hypothyroidism Is Mild Thyroid Failure and Should be Treated ‘Michael T. McDermott, E. Chester RidgwayThe Journal of Clinical Endocrinology & Metabolism October 1, 2001 vol. 86 no. 10 4585-4590
vi Hypothyroidism and chronic autoimmune thyroiditis in the pregnant state: maternal aspects.” Mandel SJ. Best Pract Res Clin Endocrinol Metab. 2004 ; 18: 213-24.
vii The role of thyroid autoimmunity in fertility and pregnancy’ Kris Poppe et al. Nature Clinical Practice Endocrinology & Metabolism(2008) 4, 394-405 (doi:10.1038/ncpendmet0846 )
viii The role of thyroid autoimmunity in fertility and pregnancy’ Kris Poppe et al. Nature Clinical Practice Endocrinology & Metabolism(2008) 4, 394-405 (doi:10.1038/ncpendmet0846 )
ix Synnøve Næss, et al., Iodine Nutrition and Iodine Supplement Initiation in Association with Thyroid Function in Mildly-to-Moderately Iodine-Deficient Pregnant and Postpartum Women, The Journal of Nutrition, Volume 151, Issue 10, October 2021, Pages 3187–3196,
x Jean M. Kerver, Et al. Prevalence of inadequate and excessive iodine intake in a US pregnancy cohort, American Journal of Obstetrics and Gynecology, Volume 224, Issue 1, 2021, Pages 82.e1-82.e8,