Subclinical hypothyroidism

Subclinical hypothyroidism means that you have higher-than-normal levels of thyroid-stimulating hormone (TSH) and normal levels of thyroid hormone thyroxine (T4). The thyroid gland is a small organ in your neck that produces hormones that help regulate vital bodily functions, including metabolism and heart rate. Up to 8% of women of reproductive age have subclinical hypothyroidism.

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Subclinical hypothyroidism is often just a temporary condition. But up to 6% of the time, it can turn into overt hypothyroidism. Hypothyroidism happens when the thyroid gland doesn’t produce enough thyroid hormones.

Impact on fertility

Research on how subclinical hypothyroidism influences female fertility is mixed. Some studies link elevated TSH to ovulation problems, unexplained infertility and diminished ovarian reserve (when most of the eggs in the ovaries have been used up). However, data is still limited. Additionally, this condition isn’t more common among women struggling to conceive than in the general population.

Thus, the American Society for Reproductive Medicine (ASRM) and American Thyroid Association (ATA) state that there isn’t enough evidence to say that subclinical hypothyroidism reduces fertility.

During pregnancy, subclinical hypothyroidism is rather common. It affects up to 28% of people. Several studies find that TSH levels above 4 mU/L are associated with negative pregnancy outcomes, including miscarriage, premature placental abruption, and newborn death, and may impair neurological development in children.

Potential causes

Subclinical hypothyroidism can be due to:

• Hashimoto thyroiditis (an autoimmune condition in which the body produces antibodies that attack the thyroid)
• Thyroid inflammation
• Iodine deficiency
• Surgery on the thyroid
• Radiation therapy at the front of the neck
• Certain medications (lithium, amiodarone, and some cancer drugs)

Symptoms

Subclinical hypothyroidism usually doesn’t cause symptoms. But some people may experience mild symptoms of hypothyroidism, such as:

• Heavy and frequent periods
• Fatigue
• Sore or cramped muscles
• Poor memory
• Dry skin and hair
• Weight gain
• Cold sensitivity
• Loss of sex drive
• Constipation

This condition is associated with other health effects, including problems with metabolism and the heart.

Diagnosis

Subclinical hypothyroidism is diagnosed based on blood tests for thyroid hormones. If your lab results show that you have elevated TSH but normal levels of T4, it signals subclinical hypothyroidism.

Treatment to boost your chances of a healthy pregnancy

Medical experts disagree on whether or not subclinical hypothyroidism should be treated because the evidence is mixed on how helpful it actually is. Healthcare providers usually recommend waiting to see if TSH levels will decrease on their own, which happens about 60% of the time.

But treatment with thyroid hormone replacement medication (levothyroxine) is recommended in the following cases:

• When TSH levels are highly elevated (over 10 mU/L)
• When subclinical hypothyroidism causes symptoms
• When someone is at risk for heart disease

Furthermore, evidence finds that women with TSH levels above 4.0 mU/L who get treatment with levothyroxine have higher rates of pregnancy and lower rates of miscarriage. And guidelines by the American Thyroid Association (ATA) state that women undergoing in vitro fertilization (IVF) or intracytoplasmic sperm injection (ICSI) should be treated with levothyroxine to decrease TSH to under 2.5 mU/L. The ATA adds that thyroid medication may help prevent subclinical hypothyroidism from turning into full-blown hypothyroidism. 

The bottom line is that deciding to treat subclinical hypothyroidism is individual, so work together with your healthcare provider to choose the course of action that’s right for you and your body. 

You can also do some things to boost your thyroid health: Make sure to get enough vitamin D, iron, and zinc and eat iodine-rich foods like seafood.

This article has been verified by a medical professional

• Subclinical Hypothyroidism. Cleveland Clinic. Accessed 22 November 2022.
• Subclinical Hypothyroidism–What Is It? And Could It Affect Fertility? Endocrine Web. Accessed 22 November 2022.
• Rao M et al.: Subclinical Hypothyroidism Is Associated with Lower Ovarian Reserve in Women Aged 35 Years or Older. Thyroid. 2020;30(1):95-105.
• How Do FSH Levels Affect Fertility? American Pregnancy Association. Accessed 22 November 2022.
• Rao M et al.: Effect of levothyroxine supplementation on pregnancy outcomes in women with subclinical hypothyroidism and thyroid autoimmuneity undergoing in vitro fertilization/intracytoplasmic sperm injection: an updated meta-analysis of randomized controlled trials. Reprod Biol Endocrinol. 2018;16(1):92.
• Maraka S et al.: Subclinical Hypothyroidism in Pregnancy: A Systematic Review and Meta-Analysis. Thyroid. 2016;26(4).
• Individualized Management of Subclinical Hypothyroidism. Cleveland Clinic Consult QD. Accessed 22 November 2022.
• Practice Committee of the American Society for Reproductive Medicine: Subclinical hypothyroidism in the infertile female population: a guideline. Fert Stert. 2015;104(3):545-553.
• Alexander P et al.: 2017 Guidelines of the American Thyroid Association for the Diagnosis and Management of Thyroid Disease during Pregnancy and the Postpartum. Clinical Thyroidology for the Public. 2017;19(5).
• Livingston EH, MD: Subclinical Hypothyroidism. JAMA. 2019;322(2):180.