Subclinical hyperthyroidism

Subclinical hyperthyroidism is a form of thyroid dysfunction characterized by low or undetectable levels of thyroid-stimulating hormone (TSH) and normal levels of thyroid hormones T3 and T4. The thyroid gland is a small organ at the bottom of your neck that helps control metabolism, heart rate, and other essential bodily functions. 
Subclinical hyperthyroidism is often only temporary. Rarely (around 5% of the time), it might turn into overt hyperthyroidism, which is when the thyroid makes too much T3 and T4.

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Impact of subclinical hyperthyroidism on fertility

There is a lack of evidence on how subclinical hyperthyroidism affects fertility. One study found that women with subclinical hyperthyroidism had higher rates of cycle cancellation when undergoing ICSI treatment. During assisted reproduction treatments IVF and ICSI, a cycle can be canceled for many reasons, including a poor reaction to hormonal medication meant to make the ovaries release more eggs. However, once the oocytes were retrieved, women with subclinical hyperthyroidism had similar rates of pregnancy and delivery as women with normal thyroid function.

Research shows that subclinical hyperthyroidism doesn’t lead to complications during pregnancy. It may even help you have a healthy pregnancy by decreasing the risk of high blood pressure. 

Overt hyperthyroidism, on the other hand, can reduce fertility by suppressing ovulation and leading to pregnancy complications.

Potential causes of subclinical hyperthyroidism

Subclinical hyperthyroidism can be due to: 

• Temporary inflammation of the thyroid caused by a viral infection
• Thyroid hormone treatment for hypothyroidism 
• Noncancerous growths on the thyroid
• Graves’ disease (an autoimmune disorder that affects the thyroid)
• Iodine deficiency

During pregnancy, subclinical hyperthyroidism is usually caused by normal changes in the body, because the pregnancy hormone beta-human chorionic gonadotropic (beta hCG) suppresses TSH.

Symptoms of subclinical hyperthyroidism

In most cases, subclinical hyperthyroidism doesn’t cause symptoms. Sometimes people with subclinical hyperthyroidism may have mild forms of hyperthyroid symptoms, such as: 

• Light, irregular, and missed periods (amenorrhea)
• Weight loss
• Fast or irregular heartbeat
• Nervousness and irritability
• Trembling hands
• Heat sensitivity
• Sleeping problems
• Sweating
• Thin hair and skin
• Weak muscles
• Enlarged thyroid 

Subclinical hyperthyroidism is also associated with osteoporosis and heart problems later in life.

Diagnosis of subclinical hyperthyroidism

Doctors diagnose subclinical hyperthyoidism based on the results of thyroid blood tests. If your tests show low or undetectable levels of TSH and normal levels of T4 and T3, it means you have subclinical hyperthyroidism. 

Treatment of subclinical hyperthyroidism

Treatment for sub-clinical hyperthyroidism can include anti-thyroid medication, radioiodine therapy, or surgery. If the low TSH levels are due to treatment for hypothyroidism, the medication dose should be adjusted. 

It’s recommended to get periodic thyroid screenings throughout life – if your TSH levels stay low over a 3-6 month period, you will likely be diagnosed with a thyroid disorder. 

To prevent the risk of bone problems due to subclinical hyperthyroidism, make sure to get enough calcium in your diet. Vitamin D, iron, and zinc are all linked to thyroid health, so you can speak to your doctor about supplementation. You can also schedule a consultation with a LEVY nutritionist to create a personalized meal plan to help boost your fertility naturally.

• Subclinical Hyperthyroidism. Cleveland Clinic. Accessed 21 November 2022.
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