TSH

The TSH test evaluates thyroid function by measuring the level of thyroid-stimulating hormone, which regulates the thyroid gland’s hormone production. Deviations in the values indicate the possibility of hypothyroidism or hyperthyroidism and help direct the diagnostic process correctly.

TSH level analysis, together with free T3 and T4 values, plays an important role in determining the severity of thyroid diseases. Monitoring changes in hormonal balance contributes to the clinical assessment of metabolic rate and energy balance.

TSH screening is especially used to detect thyroid dysfunctions early in pregnancy. Regular follow-up in expectant mothers allows timely management of hormonal imbalances that may affect fetal development, thereby improving the quality of prenatal care.

Serial TSH measurements help adjust medication doses in patients receiving thyroid treatment. Regular monitoring facilitates the evaluation of treatment response and supports clinical decision-making processes aimed at maintaining long-term thyroid health.

What Is TSH?

TSH (Thyroid-Stimulating Hormone) is a hormone secreted from the pituitary gland that stimulates the thyroid gland to produce hormones. This hormone, which regulates thyroid function, plays a critical role in balancing the body’s metabolism. TSH levels are used in the diagnosis of thyroid disorders such as hypothyroidism (low thyroid function) or hyperthyroidism (overactive thyroid function). It is assessed by measuring its level in the blood.

How do thyroid hormones affect fertility?

Thyroid hormones have an effect on every cell in the body, but they have both indirect and direct effects on the reproductive system:

The best-known indirect effect appears in cases of hypothyroidism (underactive thyroid). When thyroid hormone in the body decreases, the brain compensates by stimulating the pituitary more intensely. This stimulation increases not only TSH production but also the production of “prolactin,” the milk hormone. When prolactin, which normally should only be high in the breastfeeding period, rises due to hypothyroidism (hyperprolactinemia), it disrupts the pattern of the main reproductive hormones (FSH and LH) that are secreted from the brain to stimulate the ovaries. This can cause ovulation to become irregular or stop completely, leading to infertility.

In recent years, it has also been understood that there are direct effects. Research has shown the presence of specific “receptors” that recognize TSH and thyroid hormones in both ovarian cells and the lining of the uterus (endometrium). This means that TSH can directly affect egg quality or the ability of the uterine lining to accept the embryo.

Why does the “normal” TSH value differ from person to person?

This is the most confusing part. On your laboratory report, the “normal” range is usually written as 0.5–4.5 mIU/L (or similar). However, this “normal” range is very broad and includes, for example, a 70-year-old who is not trying to conceive.

In reproductive medicine, we cannot use these ranges as they are, because the situation is very different. Whether a TSH value is normal or not depends on many factors.

Some of these factors are:

• Age
• The laboratory method used
• Time of day
• And most importantly: Pregnancy status

Why is the TSH target set at 2.5 mIU/L when trying to conceive?

There are two main reasons for this 2.5 target. First, large population studies have shown that the average TSH level in healthy young women with no thyroid problems is around 1.5, and the majority have levels below 2.5. This led to the idea that the upper laboratory limit of 4.5 may be artificially high, because it unknowingly includes individuals with “silent” thyroid dysfunction.

The second and more important reason is the concept of mimicking pregnancy. When a healthy pregnancy begins, the pregnancy hormone secreted by the baby (hCG) has a structure very similar to TSH. This hCG stimulates the thyroid gland, like TSH, to produce more thyroid hormone. The increase in thyroid hormone in the blood sends a message to the pituitary gland saying “you don’t need to produce TSH,” and TSH is naturally suppressed.

Therefore, in a healthy pregnancy, it is expected that TSH will be below 2.5 in the first trimester. Since the aim in IVF treatment is also to prepare the body as optimally as possible for pregnancy, we try to bring the TSH level into this “ideal pregnancy” range, that is, below 2.5 mIU/L, from the very beginning.

However, is this 2.5 threshold a magical cut-off that applies to everyone? Does lowering a TSH of 3.0 to 2.0 with medication really provide any benefit? This is one of the biggest areas of debate in medicine.

How do overt thyroid disorders (Hyperthyroidism and Hypothyroidism) affect IVF treatment?

Although mild elevations of TSH are controversial, there is no debate about “overt” or fully established thyroid diseases. These conditions must be corrected before starting IVF treatment.

Overt Hypothyroidism (TSH high, T_4 low):

Markedly reduced function of the thyroid gland is very dangerous for fertility. It not only disrupts ovulation patterns but also carries serious risks even if pregnancy occurs.

Risks of untreated overt hypothyroidism:

• Miscarriage
• Preterm birth
• Preeclampsia (pregnancy-induced hypertension)
• Delayed intellectual and neurological development in the baby

Therefore, once this condition is diagnosed, thyroid medication must be started. After TSH and T_4 levels have completely normalized and remained stable for at least 4–6 weeks, IVF treatment can be started safely.

Overt Hyperthyroidism (TSH low, T_4 high):

Overactivity of the thyroid gland (also known as “toxic goiter”) is similarly dangerous. It disrupts both the menstrual cycle and ovulation.

Risks of untreated overt hyperthyroidism:

• Miscarriage
• Preterm birth
• Fetal growth restriction
• “Thyroid storm” in the mother (a life-threatening metabolic crisis)

This condition must also be controlled (usually with antithyroid drugs), thyroid function must become stable, and only then should IVF treatment be planned.

What is “Subclinical Hypothyroidism” (Silent Thyroid) and why is it so controversial in IVF?

The most common and most confusing condition encountered in clinical practice is “subclinical hypothyroidism” (SCH).

This is a laboratory-based diagnosis:

• TSH: High (usually above 4.5 but below 10)
• T_4 (the main hormone): Normal

Simply put, this is “early-stage” thyroid failure. The thyroid gland can still produce enough hormone, but only by making the pituitary gland “shout louder” (i.e., increase TSH). The thyroid is “tired,” but not yet “exhausted.”

The impact of this condition—seen in 4–8% of women of reproductive age—on fertility and IVF outcomes is one of the most contradictory areas in the scientific literature.

Does high-normal TSH (between 2.5 and 4.5) impair egg or embryo quality?

Evidence regarding the effect of TSH on the earliest stages of the IVF process, namely egg and embryo development, is divided. Some studies have suggested that as TSH levels increase, egg quality or fertilization rates decline.

However, this evidence is not universal. Many large-scale studies have found no meaningful relationship between TSH levels and the number of eggs obtained, fertilization rates or embryo quality in IVF.

This confusion likely arises from the fact that looking at TSH alone is misleading. A very important part of the picture is missing.

Does a high TSH level reduce the chance of pregnancy in IVF?

The main clinical debate concerns the effect of TSH on the final outcomes of IVF treatment (pregnancy, miscarriage, live birth). The literature is sharply divided on this topic as well.

Some studies have reported that in women with TSH > 2.5 mIU/L, especially when TSH exceeds 4.0, pregnancy rates decrease and miscarriage risk increases. Such findings have historically supported the tendency to lower TSH below 2.5 in all IVF patients.

However, more recent, larger and better-designed studies have challenged this paradigm. A large summary of studies involving thousands of women found no significant difference in clinical pregnancy or live birth rates between women with TSH < 2.5 and those with TSH between 2.5–4.5, provided that their thyroid antibody tests were negative.

This is a groundbreaking finding. It tells us that the real culprit may not be TSH itself, but rather the underlying “immune” problem that causes TSH to rise.

A factor more important than TSH: What are thyroid antibodies (TPOAb and TgAb)?

When evaluating IVF success, we now focus more on the condition known as “thyroid autoimmunity” (TAI) than on the TSH value alone. TAI is a state in which the immune system mistakenly attacks its own thyroid gland—“friendly fire.”

This attack occurs through antibodies that can be measured in the blood. The two most clinically important antibodies are:

• TPOAb (Anti-TPO): Attacks a key enzyme in thyroid hormone production (thyroid peroxidase).
• TgAb (Anti-Thyroglobulin): Attacks thyroglobulin, the protein precursor of thyroid hormone.

The presence of these antibodies is a hallmark of autoimmune thyroid disease known as Hashimoto’s thyroiditis.

Why are these thyroid antibodies more common in women with infertility?

While TAI is seen in about 10% of the general female population, its prevalence can rise to 19% among women with infertility. This association is particularly strong in women with “unexplained infertility,” “polycystic ovary syndrome (PCOS)” and “primary ovarian insufficiency (POI).” This suggests that infertility and autoimmunity may share common underlying mechanisms.

How can these thyroid antibodies prevent pregnancy even when TSH is normal?

This is the most critical point of the discussion. A woman may have a TSH of 2.0 (which appears excellent), but if her TPOAb antibodies are positive, this can still adversely affect fertility. This is called “euthyroid TAI” (normal thyroid function with autoimmunity).

These antibodies can cause harm even when TSH is normal in several ways:

• They may directly damage the ovaries.
• They may impair egg quality.
• They may create a generalized imbalance in the immune system.
• This imbalance may make it more difficult for the uterus to accept the embryo.
• They may create a “reduced thyroid reserve.”

The “reduced thyroid reserve” theory is very important. The thyroid glands of women with TAI are like “wounded soldiers.” Under normal conditions (TSH is normal) they can still perform their duties, but under increased demand or stress they may quickly become insufficient.

The high-dose estrogen used to stimulate the ovaries during IVF treatment, or pregnancy itself, dramatically increases the need for thyroid hormone. A woman whose thyroid reserve is reduced due to TAI may not be able to meet this increased demand. Just at the critical time for implantation of the embryo or early fetal development, her TSH may rise rapidly, and she may slip into hidden hypothyroidism.

Are thyroid antibodies more important than TSH for IVF success?

Evidence is increasingly pointing in this direction. Regardless of TSH level, the presence of thyroid autoantibodies may be a stronger and more reliable marker of adverse reproductive outcomes.

This is a fundamental distinction for clinical practice. Two women with the same normal TSH value (for example, 2.0 mIU/L) may have very different reproductive prognoses:

• TPOAb-negative woman: Can be considered low risk from a thyroid perspective.
• TPOAb-positive woman: Despite her normal TSH, she carries an independent risk of miscarriage and potential IVF failure due to immune system factors.

Therefore, we may be more concerned about a woman with TSH of 2.0 who is antibody-positive than about a woman with TSH of 3.5 who is antibody-negative.

Why is testing for thyroid antibodies (TPOAb) as important as TSH testing?

Because the TPOAb test acts as a “translator” for interpreting TSH results. The clinical meaning of a TSH value changes completely depending on whether TPOAb is positive or negative.

This is why it is so important to include TPOAb testing routinely, alongside TSH measurement, as part of the initial infertility evaluation. Identifying TAI (presence of antibodies) allows for more accurate risk stratification and patient counseling.

How is a treatment decision made according to TSH and antibody status?

Now we can put all the pieces together. The treatment decision is based on these two values (TSH and TPOAb). We can divide patients into four main risk groups.

• Group 1: Low Risk
• Group 2: High Risk
• Group 3: Intermediate/Uncertain Risk
• Group 4: Special Risk Group (Normal TSH, Positive Antibodies)

Here is the approach in these groups:

• Group 1 (Low Risk): TSH < 2.5 mIU/L AND TPOAb Negative

This is the ideal scenario. Thyroid function is optimal and there is no underlying immune problem. No thyroid treatment or special follow-up is required for these patients.

• Group 2 (High Risk): TSH > 4.5 mIU/L (regardless of TPOAb status)

This group includes patients with “subclinical” (TSH between 4.5–10) or “overt” (TSH > 10 or low T_4) hypothyroidism. Here the evidence is clear: treatment is necessary. Thyroid medication (LT4) is started with the goal of reducing TSH below 2.5 mIU/L and achieving a stable level before beginning IVF treatment.

• Group 3 (Intermediate/Uncertain Risk): TSH between 2.5–4.5 mIU/L

This is the most confusing “gray area.” The decision in this group depends entirely on TPOAb antibody status.

Subgroup 3a (in this range and TPOAb POSITIVE): There is a strong tendency to start treatment. These patients have a high risk of progressing to overt thyroid failure (Group 2) under the stress of IVF treatment or pregnancy. Although the evidence is not definitive, as a precaution it is a reasonable strategy to start low-dose LT4 treatment and reduce TSH below 2.5.

Subgroup 3b (in this range and TPOAb NEGATIVE): This is the most controversial subgroup. Evidence that treatment provides benefit is very weak and conflicting. There is a risk of “overtreatment” in these patients. The decision should be individualized. If the patient has additional risk factors, such as “unexplained infertility” or “recurrent pregnancy loss,” treatment may be considered. However, routinely starting medication in an antibody-negative woman with a TSH of 3.0 is not recommended.

• Group 4 (Special Risk Group): TSH < 2.5 mIU/L (Normal) BUT TPOAb POSITIVE

This group best illustrates the modern approach to treatment. Because their TSH is normal, these patients appear “healthy,” but due to their antibodies they are actually “high risk.”

There is no strong evidence that starting routine thyroid medication (LT4) in these patients increases live birth rates. The problem is immune (antibodies), not hormonal (TSH). Providing extra hormone does not stop the immune system’s attack.

In this group, the main strategy should be “precaution, not overtreatment.” Because these patients have a high risk of developing thyroid failure during pregnancy (“wounded soldier” theory), they need very close monitoring. TSH should be checked immediately before the IVF cycle and as soon as pregnancy is confirmed, and even a slight increase above 2.5 should prompt starting medication without delay.

How is thyroid medication adjusted and followed?

When LT4 treatment is started, a systematic approach is necessary to reach the therapeutic target (TSH < 2.5 mIU/L) before the start of the IVF cycle.

Usually, a low initial dose such as 25–50 mcg/day is chosen. After starting or adjusting the LT4 dose, 4–6 weeks should be allowed for blood levels to stabilize, and TSH should then be rechecked.

It is very important for the patient to have been on a stable LT4 dose and within the target TSH range for at least four weeks before beginning ovarian stimulation (the medication phase of IVF treatment).

Why should the thyroid medication dose be increased when pregnancy is achieved with IVF?

This is a practically and clinically crucial point. For a woman who is already taking thyroid medication (LT4) before conceiving, either via IVF or naturally, pregnancy immediately increases the burden on the thyroid gland. Fetal development, especially brain development, in the first trimester depends entirely on the mother’s thyroid hormones.

To meet this increased demand, women whose thyroid reserve is already reduced due to TAI or hypothyroidism must increase their external medication dose.

What should be done when pregnancy begins (only for those already on LT4):

• As soon as the home pregnancy test shows a positive result
• Without waiting for a doctor’s appointment or new blood test results
• Immediately increase the daily medication dose by about 25–30%
• (A practical way to do this: on two days of the week—for example Monday and Thursday—take twice the usual daily dose)
• Notify the clinic immediately after making this change

This proactive dose increase prevents TSH from rising to dangerous levels in the critical early weeks of pregnancy. After the dose is adjusted, TSH should be monitored closely every four weeks during the first half of pregnancy.

What is the main message in thyroid and TSH management?

The role of TSH in IVF treatment is complex, but the approach can be simplified. The key message is to avoid treating a “number” (the TSH value on the lab report) in isolation. The real “patient” to be treated is defined by the combination of their TSH value and TPOAb antibody status.

A woman with a TSH of 3.0 who is antibody-negative may be at much lower risk than a woman with a TSH of 2.0 who is antibody-positive (Hashimoto’s).

The correct approach is to avoid unnecessary treatment (over-medicalization), while staying alert and proactive in high-risk patients (especially TPOAb-positive) from the moment pregnancy begins. This balanced and individualized strategy offers the safest path to IVF success by minimizing thyroid-related problems.

Frequently Asked Questions