How does the TSH level affect fertility?
The thyroid produces hormones that influence numerous metabolic processes in the body and also play a role in the female cycle and reproduction. Pronounced thyroid dysfunction can therefore also be relevant when trying to conceive.
The most important laboratory value for an initial assessment of thyroid function is TSH (thyroid-stimulating hormone). If the value is abnormal, further tests such as measuring the free thyroid hormone fT4 and—depending on the clinical question—thyroid antibodies may be useful.
Whether a thyroid evaluation is necessary when trying to conceive depends on the individual situation. It is particularly relevant, for example, in the case of a known thyroid condition, corresponding symptoms, or abnormal previous findings.
- Thyroid, TSH, and fertility when trying to conceive
- Therapies & treatment options
- Causes of a low or high TSH level
- Symptoms of hypothyroidism
- Hashimoto’s thyroiditis and trying to conceive
- Thyroid dysfunction after pregnancy
Thyroid, TSH, and fertility when trying to conceive
TSH, T3, and T4: How thyroid function is regulated
TSH (thyroid-stimulating hormone) is produced in the pituitary gland and regulates the production of thyroid hormones. If their concentration in the blood decreases, more TSH is usually released, stimulating the thyroid to produce the hormones T4 (thyroxine) and T3 (triiodothyronine). If the concentration of thyroid hormones increases, TSH secretion decreases again.
This regulatory loop is additionally controlled by TRH (thyrotropin-releasing hormone), which is produced in the hypothalamus. Because TSH responds sensitively to changes in thyroid function, it is the most important laboratory value for an initial assessment. If TSH is abnormal, additional values—especially fT4—can be used for further classification.
Recommended TSH levels when trying to conceive and during pregnancy
The TSH level is measured in the blood and assessed based on the reference range of the respective laboratory. A single cut-off value does not apply equally to every life situation. Age, pregnancy, and an existing thyroid condition can also play a role in the assessment.
During pregnancy, thyroid function changes, among other factors, due to the influence of the pregnancy hormone hCG. According to the current German guideline, a TSH level of 4.0 mU/L is considered the upper reference limit if no suitable regional or laboratory-specific reference values for pregnant women are available. A TSH level above 2.5 mU/L therefore does not automatically mean that hypothyroidism is present or that treatment is required.
When trying to conceive, too, a TSH target value below 2.5 mU/L does not apply across the board to every woman. The situation may be different, for example, in the case of already known and treated hypothyroidism. In such cases, lower therapeutic target values may be aimed for before and during pregnancy. Classification should therefore always be based on the individual baseline situation.
Thyroid antibodies and Hashimoto’s: What matters when trying to conceive
In Hashimoto’s thyroiditis, the immune system targets components of the thyroid. Typical findings include antibodies against thyroid peroxidase (TPO antibodies). Thyroglobulin antibodies (Tg antibodies) may also occur. TRAb (TSH receptor antibodies), by contrast, mainly play a role in Graves’ disease.
However, measuring thyroid antibodies is not generally necessary for every woman when trying to conceive. What matters are the individual medical history, thyroid function, and existing findings. A positive TPO antibody result also does not automatically mean that treatment with thyroid hormones is necessary.
If TSH is elevated, free T4 (fT4) and—depending on the findings—TPO antibodies are measured in particular for further classification. Whether treatment with levothyroxine is appropriate depends, among other factors, on whether overt or subclinical hypothyroidism is present, how high the TSH level is, and whether pregnancy already exists or a thyroid condition is being treated.
For women with normal thyroid function and positive TPO antibodies, it cannot be generally concluded that preventive treatment with levothyroxine improves fertility or pregnancy outcomes. The decision should therefore be made on an individual basis.
Good to know: In Germany, testing for congenital hypothyroidism is part of newborn screening. For this purpose, among other things, TSH is measured from a dried blood spot in the first days of life.
At-home fertility test for women
Thyroid function can play a role when trying to conceive. TSH is therefore one of the seven hormone values measured by our at-home fertility test for women. In addition to TSH, AMH, FSH, LH, estradiol, prolactin, and testosterone are measured. The results are evaluated by specialist physicians and explained clearly for you—conveniently from home.
Go to the fertility test
Hypothyroidism and subclinical hypothyroidism: Therapies & treatment options
Hypothyroidism is usually treated with levothyroxine (L‑thyroxine). The medication replaces the thyroid hormone T4, which the body does not produce in sufficient amounts in the case of underactivity. The required dose is adjusted individually based on thyroid values.
In subclinical hypothyroidism—i.e., an elevated TSH level with normal fT4—treatment is not required in every case. Whether levothyroxine is appropriate depends, among other factors, on the level of TSH, additional thyroid findings, and whether pregnancy exists or is planned.
Women with already known and treated hypothyroidism should have their thyroid values checked, if possible, before a planned pregnancy. If there is an indication for treatment, levothyroxine can and should also be continued during pregnancy. Because the need for thyroid hormones often increases during pregnancy, regular monitoring is important and the dose may need to be adjusted.
Causes of a low or high TSH level
A TSH level outside the reference range may indicate altered thyroid function. For further classification, the free thyroid hormones—especially fT4—are taken into account.
A low TSH level may indicate hyperthyroidism. Possible causes include, for example:
- Graves’ disease
- thyroid autonomy, for example due to autonomous nodules
Graves’ disease is an autoimmune condition in which antibodies against the TSH receptor (TRAb) stimulate the thyroid and can thereby trigger increased production of thyroid hormones. In addition to the thyroid, the condition can also affect other organs, especially the eyes.
An elevated TSH level, by contrast, may indicate hypothyroidism. A common cause is Hashimoto’s thyroiditis, an autoimmune disease of the thyroid. However, whether there is actually hypothyroidism requiring treatment does not depend on the TSH level alone.
Symptoms of hypothyroidism
Hypothyroidism can present with various, often non-specific symptoms. Especially when trying to conceive or planning a pregnancy, it can be important to detect an existing thyroid condition early and treat it if necessary.
Possible symptoms of hypothyroidism include:
- Fatigue
- dry skin
- constipation
- muscle or joint pain
- increased sensitivity to cold and feeling cold more often
- concentration and memory problems
- changes in the menstrual cycle
Because these symptoms can also have many other causes, hypothyroidism cannot be diagnosed based on symptoms alone. For clarification, thyroid values are measured in the blood.
Hashimoto’s thyroiditis and trying to conceive
Hashimoto’s thyroiditis is an autoimmune disease of the thyroid. The immune system targets components of thyroid tissue. Over time, thyroid function can decline and hypothyroidism may develop. Typical findings include elevated TPO antibodies; thyroglobulin antibodies (Tg antibodies) may also be detectable.
Hashimoto’s is relatively common in women of reproductive age and can therefore also play a role when trying to conceive. Thyroid dysfunction can affect the cycle and fertility, among other things. Studies also show associations between autoimmune thyroid disease and various reproductive factors. However, the extent to which thyroid autoimmunity—independent of thyroid function itself—affects fertility has not yet been conclusively clarified.
When trying to conceive, the key factor is therefore primarily whether the thyroid produces sufficient hormones. If Hashimoto’s is associated with hypothyroidism requiring treatment, it is treated with levothyroxine, and thyroid levels should be checked already before pregnancy. Positive thyroid antibodies alone, by contrast, do not automatically mean that treatment with levothyroxine is necessary.
Postpartum thyroiditis: Thyroid dysfunction after pregnancy
Postpartum thyroiditis is an inflammatory autoimmune disease of the thyroid that can occur within the first year after childbirth. It affects about 5–10% of women after pregnancy; it is more common in certain risk groups.
Typically, an initial phase with elevated thyroid hormone levels may occur, followed by a phase of hypothyroidism. However, not all women go through both phases. Some experience only a hyperthyroid or only a hypothyroid phase.
In the early phase, symptoms such as palpitations, restlessness, sleep disturbances, or weight loss may occur. In the later hypothyroid phase, fatigue, weight gain, constipation, dry skin, or depressive mood are possible. Because some of these symptoms are also common in the postpartum period, the condition is not always recognized immediately.
Immunologically, postpartum thyroiditis resembles Hashimoto’s thyroiditis and is often associated with TPO antibodies. Whether treatment is necessary depends on the phase and the severity of symptoms. In many women, thyroid function normalizes again within 12 to 18 months; however, in some, permanent hypothyroidism persists.
Korevaar TIM, Leung AM, Alexander EK et al. American Thyroid Association 2026 Guidelines for Thyroid Disease in Preconception, Pregnancy, and Postpartum. Thyroid. 2026;36(5):481–544. doi: 10.1177/10507256261445624.
DEGAM / AWMF. S2k guideline “Elevated TSH level in general practice”, AWMF register number 053–046.
Poppe K, Bisschop P, Fugazzola L et al. 2021 European Thyroid Association Guideline on Thyroid Disorders prior to and during Assisted Reproduction. European Thyroid Journal. 2021;9(6):281–295. doi: 10.1159/000512790.
PoppeK,Bisschop PH, Fugazzola L, Mintziori G, Unuane D & Weghofer A European Thyroid Association guideline on thyroid disorders prior to and during assisted reproduction. European Thyroid Journal 2020 9 281–295.
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