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Thyroid & Fertility

Thyroid disorders are crucial fertility conditions that are easily overlooked, especially hypothyroidism (an underactive thyroid) which is more common than hyperthyroidism. It has a particularly significant effect on female fertility, with women more likely to have 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 “unknown infertility

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 systems are complex, and the hypothalamus and pituitary glands regulate the thyroid, adrenal and reproduction systems: iv

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

The primary connections between thyroid function and fertility are:

  1. Specific thyroid hormone receptors on the ovaries alter how the ovaries 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”. When prolactin is at a high level, it will stop menstrual cycles (hyperprolactinaemia)
  4. Low T3 and T4 levels affect ovarian function by reducing sex-hormone-binding globulin (SHBG) levels and increasing prolactin and TSH from the anterior pituitary. The outcome is lower FSH and LH levels, and they’re essential for menstrual cycles to start

Subclinical Hypothyroidism

T4 levels are normal in the subclinical cases, but the differences in the hormone half-life reveal the body is struggling:

  • The half-life of T4 is 7 days
  • The half-life of T3 is 1 day
  • The half-life of serum TSH has a half-life of less than 1 hour

In a balanced thyroid axis, TSH levels quickly return to normal once T4 has been released, as T4 lasts much longer than TSH. However, in subclinical hypothyroidism, the high TSH levels indicate the thyroid needs abnormal levels of stimulation to maintain normal T4 levels.v This is a particular concern when trying to conceive or sustain a pregnancy as the body has an increased need for thyroid hormones (especially from weeks 6 to 20) and stays relatively high for months after the birth.

Subclinical hypothyroidism is often an early stage of the overt condition, and 2-5% of subclinical cases will become overt each year. However, it’s also possible for the condition to resolve or remain unchanged.

Causes of hypothyroidism

  1. A lack of iodine in the diet is the most common cause of hypothyroidism in the world
  2. Where iodine is sufficient, chronic inflammation of the thyroid from an autoimmune disease is the usual cause of hypothyroidism vi

Stress contributes to both immune diseases and thyroid dysfunction, and it can come from:

  1. Environmental stress from exposure to toxins, poor diet or extremes of climate
  2. Fluctuating blood sugar levels
  3. Immune problems leading to autoimmune illness and autoimmune thyroid diseases are estimated to be 5-10 times more common in women than men
  4. Adrenal stress

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

Hypothyroidism and ART

When women with low thyroid levels are stimulated in assisted reproductive techniques (ART), it increases 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. This link is especially valid for women whose infertility includes endometriosis or premature ovarian failure viii

Treatment of hypothyroidism

  • Treatment aims to 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 will allow TSH and prolactin levels to return to normal. The pituitary then releases enough follicle-stimulating hormone (FSH) to trigger ovulation
  • Fertility, as well as energy levels, will rise, and ART is often 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. A daily dose of 25 to 75μg is usual (depend­ing on the TSH elevation), with later adjustments often needed
  2. Iodine (and other mineral) supplements needed for thyroxine (T4) production and its conversion to T3. The conversion of T4 to T3 is in the liver and muscles and 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. Poor conversion of T4 to the far more active T3 only shows up in tests that assess both hormones
  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. Hypothyroidism typically generates a Cold PFP, which can affect the fertility of both sexes. We recommend following personal fertility profile advice, as well as taking specific action to address thyroid function

For excellent advice and research information, see Thyroid UK.