Learn how hyperthyroidism is diagnosed and how it might impact fertility.

What is hyperthyroidism, and is it a cause of infertility? This article explores current research and knowledge to help clarify the relationship between this condition and reproductive health.

Introduction

Hormones act as chemical messengers in the body. They are produced by endocrine glands and travel through the blood to the body's organs and tissues, where they impact various processes, including:

  • Growth and development
  • Metabolism
  • Mood
  • Sexual function
  • Sexual reproduction (i.e., fertility)

Hyperthyroidism is a medical condition involving excessive concentrations of thyroid hormones in bodily tissues. It affects around 1% of people, with some variance by geographic region. It is more common in women than in men and appears more frequently with increasing age.

In this article, we will help you understand how hyperthyroidism is defined and its symptoms. We will then look at how the condition is diagnosed and treated and how this alteration in thyroid function might impact fertility and attempts to conceive.

What is hyperthyroidism?

The thyroid gland is a bodily organ vital to hormone-related functions such as metabolism, growth, and development. It is butterfly-shaped and located at the bottom of the front of the neck, below the voice box, and above the collarbone. It is relatively small, typically weighing between 20 and 60 grams.

This gland produces three hormones:

  • Triiodothyronine (also called T3)
  • Tetraiodothyronine (also called thyroxine or T4)
  • Calcitonin

Of these three, T3 and T4 are directly related to metabolism. Their production is regulated by thyroid-stimulating hormone (TSH), produced in the brain by the pituitary gland. TSH stimulates the thyroid gland to produce thyroxine (T4), which the body converts into triiodothyronine (T3). It is T3 that then directly influences functions relating to the body's basal metabolic rate, including:

  • Body temperature
  • Heart and pulse rate
  • Brain development (in children and adolescents)
  • Growth of the body (in children and adolescents)
  • Use of food by the body

Normally, TSH and T4/T3 operate in a negative feedback loop, such that TSH production is increased when blood concentrations of T4 and T3 become low and decreased when blood concentrations of T4 and T3 become high. Abnormalities in the system do occur, however, in two primary forms:

  • In hypothyroidism, thyroid hormones are underproduced.
  • In hyperthyroidism, sometimes called overactive thyroid, thyroid hormones are overproduced.

Both types of thyroid disorders lead to disturbances in metabolic functions.

The excessive thyroid hormone concentrations in hyperthyroidism are most often caused by Graves' disease, an autoimmune condition in which thyroid-stimulating antibodies activate thyroid-stimulating hormone (TSH) receptors. These antibodies mimic TSH, which produces excessive hormone production by the thyroid.

Other causes of hyperthyroidism include:

  • Painless or transient (silent) thyroiditis, which is an autoimmune condition leading to the destruction of thyroid tissue and the overproduction of thyroid hormones.
  • Toxic adenoma, in which a nodule or tumor in the thyroid gland overproduces thyroid hormones.
  • Toxic multinodular goiter, or Plummer disease, which involves multiple thyroid nodules that overproduce thyroid hormones.

There are other, less common causes of hyperthyroidism, such as struma ovarii, an ovarian growth that contains thyroid tissue and can produce thyroid hormones. Hyperthyroidism can sometimes also appear during pregnancy – especially in conjunction with hyperemesis gravidarum – or postpartum. Hyperthyroidism during or post-pregnancy occurs due to metabolic changes during these periods. It is usually self-limiting, tending to resolve on its own.

What are the symptoms?

In general, hyperthyroidism leads to an increase in metabolic activity in the body. This can manifest in the following symptoms:

  • Weight loss despite increased appetite
  • Diarrhoea or increased GI motility
  • Heart palpitations
  • Tremors
  • Weakness
  • Heat intolerance
  • Diaphoresis (excessive sweating)
  • Menstrual irregularities
  • Anxiety

In addition, Graves' disease can have specific signs and symptoms, including:

  • Swelling of tissue around the eyes, leading to a bulging appearance (orbitopathy)
  • Skin lesions on the lower leg (pretibial myxedema)
  • Swelling of fingers and toes (thyroid acropathy)

A person with hyperthyroidism may or may not have an enlarged thyroid, known as goiter. When this enlargement does occur, it can be either diffuse (across the entire structure) or be limited to one of its nodules. The thyroid gland may also be painless when palpitated or extremely tender.

What are the risk factors?

As with other autoimmune diseases, family history is a risk factor for Graves' disease. However, being female also increases the risk, as does smoking. Obesity is associated with decreased risk of Graves' disease, but this may be due to the fact that weight loss occurs as one of its symptoms.

What is the impact of hyperthyroidism on fertility?

Hyperthyroidism can impact both female and male fertility. Studies have found that autoimmune thyroid disease (such as Graves' disease) is more common in women experiencing infertility than women without fertility problems. A link with endometriosis as a specific cause of female infertility has also been observed.

Hyperthyroidism is associated with changes in sex hormones, such as estrogen, and irregularity of the menstrual cycle, which can make conception less likely, for example, through the reduced frequency of ovulation. It is also thought that hyperthyroidism may impact fertility through compromised egg (oocyte) quality or disruptions in the process whereby an embryo implants into or begins to develop in the uterus. Both thyroid hormone receptors and TSH receptors are present in the endometrium, or uterine lining, where implantation occurs.

Hyperthyroidism in women has also been associated with an increased risk of miscarriage and pregnancy loss, both that which occurs very early, termed spontaneous abortion, and that occurring late in pregnancy, termed stillbirth.

In a cohort study of women in Denmark undergoing assisted reproductive technology treatments including in vitro fertilisation (IVF), researchers found that women with hyperthyroidism had a decreased chance of having a live birth following the treatments, which was thought to be due to disruptions at the implantation stage. By contrast, women in the study with hypothyroidism, or underactive thyroid, did not have reduced chances of live birth, but their babies were more likely to be born with congenital malformations.

In men, hyperthyroidism can lead to reduced semen quality, which includes reductions in the following:

  • Volume of semen
  • Sperm density (sperm count)
  • Sperm motility (ability to move)
  • Sperm morphology (size and shape)

As these factors make conception less likely, thyroid hormone levels may be tested for men who are part of a couple experiencing infertility and considering infertility treatment, especially if symptoms of hyperthyroidism are present.

It is important to note that reversing thyroid dysfunction by restoring thyroid hormone levels to their ideal range (known as euthyroidism) can often reverse problems with fertility caused by hyperthyroidism.

How is it diagnosed?

The first step in diagnosing hyperthyroidism is a blood test for thyroid-stimulating hormone (TSH). When the TSH level is abnormal, this test is usually followed by an assessment of free thyroxine (T4) and total triiodothyronine (T3) in the blood. The typical pattern seen in hyperthyroidism, also known as 'overt hyperthyroidism', is

  • Suppressed TSH (<0.1 milli-international units per milliliter, or mlU per mL) and
  • Elevated levels of T3 and T4.

Individuals may also be diagnosed with 'subclinical hyperthyroidism', in which

  • TSH may be only low (between 0.1 and 0.4 mlU per mL) and
  • Levels of T3 and T4 may still be within the normal range

Individuals with subclinical hyperthyroidism may experience the same symptoms as those with overt hyperthyroidism.

Following the laboratory tests outlined above, the following steps can be taken to determine the cause of hyperthyroidism:

  • Radioactive iodine uptake test: in this test, a person takes a pill or liquid containing a small amount of radioactive iodine. At intervals in the following 24 hours, a gamma probe is held to the outside of the neck to measure how much of the thyroid's radioactive iodine has been absorbed. Uptake of iodine is usually low in patients with thyroiditis and high in patients with Graves' disease, toxic multinodular goiter, or toxic adenoma.
  • Thyroid scan: in the case of high uptake, a scan can be carried out to see the distribution of radioactive iodine within the thyroid. Homogeneous distribution across the thyroid is an indication of Graves' disease. At the same time, the accumulation of iodine in one or more areas indicates either toxic adenoma or toxic multinodular goiter.

Sometimes, an ultrasound may be performed instead of a radioactive iodine uptake test and scan, especially for pregnant or breastfeeding women.

How is it treated?

When left untreated over a long period, hyperthyroidism can lead to serious health risks, making it important to seek treatment. In particular, Graves' disease, toxic adenoma (a single large thyroid lump), and toxic multinodular goiter (multiple thyroid lumps) can sometimes cause a severe form of hyperthyroidism known as a thyroid storm. This can result in the following:

  • High body temperature
  • Central nervous system effects, like delirium, lethargy, or seizure
  • Moderate to severe gastrointestinal dysfunction (nausea, vomiting, or pain)
  • Liver dysfunction, including jaundice
  • Tachycardia, or rapid heartbeat
  • Atrial fibrillation (irregular and often fast heart rate)
  • Congestive heart failure

The pharmacologic (medication-based) treatment approach to hyperthyroidism usually involves the use of

  • Beta-blockers, such as atenolol or propranolol. These medications work rapidly to control the metabolic symptoms associated with hyperthyroidism. They do not generally address the condition's underlying causes; propranolol has some effect in blocking the conversion of T4 to T3, but this is limited.
  • Antithyroid medications, known as thioamides, such as methimazole or propylthiouracil. These medications block the synthesis of thyroid hormones. They do not cure hyperthyroidism but can be taken long-term or used as a bridge to other treatments, such as radioactive iodine ablation, discussed below.

For Graves' disease, there are three main treatment options:

  • Antithyroid medication supplements: methimazole is usually the medication of choice; however, propylthiouracil is preferred during pregnancy because methimazole has been associated with an increased risk of congenital disabilities. The two medicines are equally effective.
  • Radioactive iodine ablation: the destruction of thyroid tissue using radioactive iodine. This is not approved for pregnant women. Additionally, some individuals undergoing this treatment will subsequently develop hypothyroidism and require long-term thyroid hormone replacement.
  • Thyroidectomy, or surgical removal of all or part of the thyroid gland. This treatment is done primarily for patients who cannot use or do not respond to antithyroid medications or radioactive iodine ablation.

In the case of toxic adenoma or toxic multinodular goiter, radioactive iodine ablation and thyroidectomy are the main treatment options. However, thyroiditis usually resolves on its own within six months, so only beta blockers are often used for symptom control.

Commonly Asked Questions

If I have hyperthyroidism, will I still be able to get pregnant?

Untreated hyperthyroidism may pose particular challenges to conception. However, research suggests that when hyperthyroidism is addressed through treatment, and normal thyroid functioning is restored, problems with infertility will tend to reverse themselves. Therefore, in cases where hyperthyroidism is known or suspected, treating it may help to improve a couple's chance of conceiving. A reproductive endocrinologist or other healthcare provider can help to clarify the options.

If I get pregnant with hyperthyroidism, will I have a healthy pregnancy?

Some research links hyperthyroidism to an increased risk of pregnancy loss. However, there are options to control hyperthyroidism during pregnancy. Even if discovered at this stage, it can be addressed to restore normal hormone levels and reduce the risks associated with the condition.

Takeaway

Hyperthyroidism involves the overproduction of thyroid gland hormones, leading to symptoms such as

  • Weight loss.
  • Heat intolerance.
  • Heart palpitations.

It is diagnosed via blood tests of thyroid-stimulating hormone (TSH), thyroxine (T4) and triiodothyronine (T3), and a radioactive iodine uptake test and thyroid scan can further clarify its cause. The most common cause of hyperthyroidism is the autoimmune process in Graves' disease. Still, other causes, such as thyroid nodules, also exist. Treatment can involve medication and/or reduction of thyroid tissue via radioactive iodine or surgery.

Hyperthyroidism can impact female fertility, through irregular menstruation, compromised egg quality and disruptions in the implantation process, and male fertility, through decreased semen quality. It is also associated with an increased risk of pregnancy loss. However, research suggests that when normal thyroid hormone levels are restored through treatment, problems with infertility relating to this condition tend to be reversed.

At Cada, our caring team puts individuals at the centre of their fertility care. Would you like to discuss this issue or others concerning reproductive health and fertility? Secure your free consultation and get in touch with us so that we can figure out how best to help you.