Thyroid stimulating hormone (TSH) is a hormone produced by the pituitary gland that regulates the thyroid. TSH stimulates the thyroid to produce thyroid hormones T3 and T4 which regulate metabolism in the body. Low TSH levels indicate an overactive thyroid (hyperthyroidism) that can lead to detrimental health effects if left untreated.
What causes low TSH levels?
Some common causes of low TSH include:
- Graves’ disease – An autoimmune disorder where the body produces antibodies that stimulate the thyroid, leading to excessive hormone production.
- Toxic nodules – Benign lumps on the thyroid that overproduce thyroid hormones.
- Inflammation of the thyroid (thyroiditis) – Usually a temporary condition caused by a virus.
- Too much iodine intake – Consuming foods or supplements high in iodine can overstimulate the thyroid.
- Medications – Such as too high a dose of synthetic thyroid hormone medication.
In rarer cases, low TSH can be caused by:
- A pituitary tumor producing excess TSH.
- Resistance to TSH due to an abnormal thyroid gland.
- A temporary illness, such as a severe infection.
Symptoms of low TSH
Common symptoms of having a low TSH level include:
- Unexplained weight loss
- Rapid heartbeat
- Trembling, tremors
- Sweating, heat intolerance
- Anxiety, irritability
- Muscle weakness
- Hair loss
- Loose stools
- Trouble sleeping
- Fatigue
Some people may experience more severe symptoms like chest pain, vision problems, and swelling in the neck from an enlarged thyroid. Women may have irregular or light periods. Symptoms in infants and children can include poor weight gain and diarrhea.
Dangers of untreated low TSH
Leaving low thyroid levels untreated can lead to several complications:
Cardiovascular effects
Excess thyroid hormones increase heart rate and blood pressure, placing extra strain on the heart. This raises the risk of:
- Atrial fibrillation – An abnormal heart rhythm
- Heart failure
- Heart attack
- Stroke
People with hyperthyroidism have a 3-4 times higher risk of developing atrial fibrillation compared to the general population.
Thyroid storm
In rare cases, severely untreated hyperthyroidism can trigger thyroid storm – a sudden worsening of symptoms that is a medical emergency. It is caused by a major surge in thyroid hormones in the body and can result in high fever, racing pulse, nausea, diarrhea, seizures, and coma. Thyroid storm has a mortality rate of 10-30% if not treated promptly.
Bone loss
Having excess thyroid hormones leads to increased bone turnover and loss of bone mass over time. This raises the risk of osteoporosis and fractures later in life.
Eye disease
Ongoing hyperthyroidism can cause eye problems like:
- Dryness and irritation of the eyes
- Vision problems
- Retraction of the upper eyelids
- Swelling around the eyes
- Bulging eyes (in severe cases)
If left untreated, severe cases can permanently damage eye muscles and vision.
Thyroid gland enlargement
Persistent stimulation of the thyroid gland can cause gradual enlargement of the gland, forming a goiter in the neck which can cause swallowing or breathing difficulties.
Pregnancy complications
Untreated hyperthyroidism increases the chance of:
- Preterm birth
- Low birth weight
- Preeclampsia
- Miscarriage
- Congenital abnormalities in the baby
Thyroid levels need to be monitored and controlled in pregnant women.
Growth and developmental delays in children
When hyperthyroidism develops in infants and children, the excess hormones can result in rapid bone maturation and premature fusion of growth plates. This can lead to stunted growth and short stature.
Diagnosing low TSH
If hyperthyroidism symptoms are present, a doctor will check TSH levels through a blood test. Normal TSH levels are around 0.5-4.5 mIU/L.
Low TSH levels indicate hyperthyroidism:
- Mildly low: 0.1 – 0.4 mIU/L
- Moderately low: 0.01 – 0.1 mIU/L
- Severely low: Less than 0.01 mIU/L
The lower the TSH level, the more severe the hyperthyroidism. Doctors may also test levels of T3 and T4 and check for antibodies related to autoimmune conditions like Graves’ disease. Imaging like an ultrasound, radioiodine scan, or thyroid uptake test helps identify the underlying cause.
Treatment for low TSH
There are three main treatment options for hyperthyroidism:
1. Antithyroid medications
Drugs like methimazole and propylthiouracil block thyroid hormone production. They take several weeks to normalize thyroid levels and require regular monitoring. Side effects can include rash, itching, and liver problems.
2. Radioiodine therapy
This involves taking a radioactive iodine pill that accumulates in the thyroid and destroys overactive cells. Over months, thyroid functioning decreases. It often leads to permanent hypothyroidism requiring daily thyroid medication. Regular follow-up is needed to prevent over-treatment.
3. Surgery
Removing part or all of the thyroid through surgery is an option if other treatments fail. Risks include damage to vocal cords and parathyroid glands. Lifelong thyroid hormone replacement is required after full removal.
Beta-blockers like propranolol help control symptoms like rapid heart rate until thyroid levels stabilize. Supportive treatment includes staying hydrated, cooling measures for heat intolerance, and antacids for nausea.
What happens if low TSH persists?
If left completely untreated, very low TSH levels lead to worsening hyperthyroidism symptoms. Heart, eye, and bone complications become more likely over time.
In older people, untreated severe hyperthyroidism can double the risk of mortality compared to those receiving treatment.
With treatment, TSH levels gradually increase back to normal over weeks to months and symptoms resolve. Doctors monitor TSH and thyroid hormone levels to ensure adequate control.
Lifelong follow-up is needed for conditions like Graves’ disease which can relapse. Permanent hypothyroidism is common after surgical or radioiodine treatment.
Can low TSH resolve on its own?
In some cases, low TSH resolves without treatment:
- Subacute thyroiditis – Low TSH due to inflammation often resolves within 12-18 months.
- Postpartum thyroiditis – Thyroid dysfunction after pregnancy usually normalizes within 6-12 months.
- Silent thyroiditis – Can follow a course of hyperthyroidism followed by hypothyroidism and return to normal thyroid function.
- Exposure to excess iodine – Low TSH induced by high iodine intake resolves weeks after iodine exposure ends.
However, monitoring is needed even with self-resolving causes as some may eventually develop permanent thyroid dysfunction.
Graves’ disease, toxic nodules, and tumors causing hyperthyroidism do not resolve without treatment. Allowing TSH levels to remain low in these cases leads to worse outcomes.
Prevention of low TSH
It’s not always possible to prevent the conditions that cause low TSH. However, the following can lower risk:
- Avoid smoking – Reduces the chance of Graves’ disease and thyroid inflammation.
- Moderate iodine intake – Don’t take high-dose supplements without medical supervision.
- Treat viral infections promptly – To reduce chances of temporary thyroiditis after an illness.
- Avoid thyroid-disrupting chemicals – Such as lithium, amiodarone, interferon alpha.
- Check for thyroid dysfunction after pregnancy – Up to 8% of women develop postpartum thyroiditis.
Those with a family history of autoimmune thyroid disease should be screened periodically. Diagnosing and treating hyperthyroidism early prevents complications.
Conclusion
Allowing TSH levels to remain low leads to a wide range of adverse effects over time, especially on the heart, bones, eyes and metabolism. While some transient causes of low TSH will self-resolve, most require treatment to control thyroid hormone levels. Close monitoring and follow-up care are needed to avoid under or overtreating hyperthyroidism. With proper management, a low TSH level can be corrected before permanent damage occurs.