What is TSH?
Thyroid stimulating hormone (TSH) is a hormone produced by the pituitary gland in the brain. TSH tells the thyroid gland how much thyroid hormone to make and release into the bloodstream. The main thyroid hormones are T4 (thyroxine) and T3 (triiodothyronine).
When TSH levels are high, it means the body is signaling the thyroid to produce more thyroid hormone. When TSH levels are low, it means less thyroid hormone is needed.
What is the normal range for TSH?
In healthy individuals not taking any thyroid medication, normal TSH levels generally fall between 0.4 and 4.0 mIU/L. However, the optimal TSH level differs slightly between individuals and may change over time.
Some general TSH reference ranges are:
| TSH Level | Indication |
| 0.4 – 2.5 mIU/L | Normal |
| 2.5 – 4.0 mIU/L | Mild hypothyroidism |
| 4.0 – 10 mIU/L | Hypothyroidism |
| Less than 0.4 mIU/L | Hyperthyroidism |
However, the optimal TSH target may be lower for some individuals, such as those with symptoms of hypothyroidism even when TSH is within the normal laboratory range.
Why check TSH levels on levothyroxine?
Levothyroxine is a synthetic version of the T4 thyroid hormone. It is commonly prescribed to treat hypothyroidism, a condition where the thyroid gland does not produce enough thyroid hormone on its own.
By taking levothyroxine daily, hypothyroid patients can return their thyroid hormone levels to normal. This helps relieve symptoms of low thyroid such as fatigue, weight gain, dry skin, and feeling cold.
Checking TSH levels while on levothyroxine therapy allows your doctor to ensure the dose is optimal. Taking too much levothyroxine can suppress TSH and lead to hyperthyroidism. Taking too little can lead to ongoing hypothyroid symptoms.
Target TSH range on levothyroxine
When taking levothyroxine, the goal is to normalize the TSH level within the reference range. However, there is some debate around the optimal target range.
Some major medical organizations provide the following TSH targets for levothyroxine therapy:
| Medical Organization | Recommended TSH Range on Levothyroxine |
| American Thyroid Association | 0.5-2.0 mIU/L |
| American Association of Clinical Endocrinologists | 0.3-3.0 mIU/L |
| The Endocrine Society | 0.4-4.0 mIU/L |
As shown, recommendations vary slightly between a tighter range (0.5-2.0 mIU/L) and a broader laboratory normal range (0.4-4.0 mIU/L).
Some practitioners argue for a lower TSH target between 0.5-2.0 mIU/L because this may relieve subtle hypothyroid symptoms in some patients who still have symptoms when TSH is within the upper end of the normal range.
However, other organizations advise that there is insufficient evidence that targeting a lower TSH offers benefits compared to remaining within the full normal range. More clinical studies are still needed.
Symptoms with a low but normal TSH
Sometimes patients taking levothyroxine can have a “low but normal” TSH between 0.4-2.0 mIU/L and still experience hypothyroid symptoms such as:
– Fatigue
– Weight gain
– Muscle aches
– Memory problems
– Depression
– Constipation
– Dry skin and hair
Several theories may explain ongoing symptoms when the TSH is within range:
– The optimal TSH level varies between individuals based on factors like age, gender, and genetics. Some people feel best at the lower end of the normal range, around 1.0-2.0 mIU/L.
– There may be slight inaccuracies in TSH lab tests, leading some people to be undertreated if the result is at the higher end of normal.
– Symptoms may be caused by reduced T3 levels, as levothyroxine only contains T4. Some patients benefit from adding T3 medication.
– Other underlying issues may be mistaken for hypothyroidism, like adrenal fatigue, hormonal imbalances, anemia, or vitamin deficiencies.
If you don’t feel well on levothyroxine therapy, it’s worth discussing with your doctor even if your TSH is within range. You may benefit from dosage adjustments, additional lab tests, or thyroid hormone types like T3 medication.
Why is my TSH still high on levothyroxine?
If your TSH level remains elevated above the reference range on levothyroxine, there are several possible reasons:
Inadequate dose
The most common reason for a high TSH on levothyroxine is an insufficient dosage, meaning you need a higher amount to properly replace your thyroid hormone levels.
Doctors may start levothyroxine at a low dose and slowly titrate up over several weeks based on TSH levels. But sometimes the final dose is not high enough, often from a hesitation to prescribe higher thyroid hormone doses.
Poor absorption
Stomach and gut issues can prevent full absorption of oral levothyroxine. This includes:
– Celiac disease
– Inflammatory bowel disease
– Gastric bypass surgery
– Lactose intolerance
– Gastroparesis
Vitamin and mineral deficiencies, like vitamin B12 and iron deficiency, may also impact levothyroxine absorption.
Medication interactions
Some medicines can increase levothyroxine clearance from the body, leading to lower thyroid hormone levels:
– Cholesterol medications like statins and gemfibrozil
– Seizure drugs like phenytoin and carbamazepine
– Rifampin for tuberculosis
Supplements like calcium and iron may also impair levothyroxine absorption when taken at the same time.
Non-compliance
Forgetting doses, taking levothyroxine irregularly, or stopping treatment altogether can obviously result in persistently high TSH levels.
Underlying conditions
Rarer causes of elevated TSH on levothyroxine include:
– Pituitary gland dysfunction, leading to insufficient TSH production
– Resistance to thyroid hormone action in tissues
– Thyroid hormone autoantibodies interfering with treatment
Tips for correcting a high TSH on levothyroxine
Here are some ways to optimize your levothyroxine therapy if TSH remains above target:
– **Increase the dose**: Require higher doses at 25-50mcg increments until TSH normalizes. Titrate carefully and recheck TSH in 6 weeks after each change.
– **Change timing**: Take on an empty stomach 30-60 minutes before food and coffee. Avoid interacting supplements.
– **Switch brands**: Levothyroxine absorption varies between manufacturers. May help to change brands.
– **Split dose**: Take half the daily dose in the morning and evening to minimize spikes and drops in levels.
– **Change mode**: Request liquid or injectable levothyroxine if intestinal issues impair absorption of pills.
– **Add T3**: Combination T4/T3 therapy (such as natural dessicated thyroid) may be more effective in some patients.
– **Test for causes**: Evaluate for pernicious anemia, celiac disease, SIBO, or genetic polymorphisms affecting levothyroxine metabolism.
– **Consider alternatives**: Radioactive iodine or thyroid surgery if high TSH persists despite medication adjustments.
Can you have a normal TSH and still be hypothyroid?
It is possible in some cases to have a TSH level within the healthy reference range and still experience symptoms of hypothyroidism. Potential reasons include:
– **Individual variation**: The optimal TSH level differs slightly between people. Some feel best below 2.0 mIU/L.
– **Isolated T3 deficiency**: TSH and T4 levels can be normal while T3 hormone is low, as levothyroxine only contains T4.
– **Thyroid antibodies**: Hashimoto’s disease leads to fluctuating thyroid levels, so TSH testing gives variable results.
– **Abnormal thyroid binding proteins**: Dysfunctional thyroid binding globulins can cause lab result inaccuracies.
– **Pituitary dysfunction**: Secondary hypothyroidism from pituitary gland impairment may show a “normal” TSH.
– **Genetic variations**: Differences in deiodinase enzymes that convert T4 to T3 impact tissue thyroid levels.
– **Drug interactions**: Some medications artificially increase or decrease serum TSH levels.
– **Non-thyroidal illness**: Severe illness can suppress TSH and thyroid function, masking underlying hypothyroidism.
Next steps if symptomatic with a normal TSH
If you have hypothyroid symptoms but TSH is within range, discuss further evaluation with your doctor:
– Evaluate T3, T4, thyroid antibodies, and reverse T3 levels – not just TSH.
– Assess for nutrient deficiencies impairing thyroid function – like iron, vitamin D, selenium.
– Check for related autoimmune disorders like celiac disease.
– Consider genetic testing related to deiodinase enzymes.
– Change brands of levothyroxine if levels fluctuate.
– Trial combination T4/T3 therapy or natural dessicated thyroid medication.
– Recheck TSH soon if non-thyroidal illness present when tested.
Should TSH be rechecked when on a stable levothyroxine dose?
Medical guidelines generally recommend rechecking TSH levels every 6-12 months once a stable levothyroxine dose has been established:
– It ensures the dose continues meeting your needs long-term. Thyroid requirements may increase with age.
– Changing health conditions and new medications can affect thyroid levels. Regular TSH testing identifies needs for dose adjustments.
– Even generic levothyroxine doses from the same manufacturer can vary between batches. TSH monitoring confirms consistent potency.
– If doses are borderline, more frequent TSH testing helps prevent under- or over-treatment.
– Screening for abnormal TSH recognizes emerging thyroid issues like Graves’ disease hyperthyroidism.
More frequent TSH testing, every 3-6 months, benefits patients:
– With mild hypothyroidism
– Who are pregnant
– Whose doses are being titrated
– Also on T3 therapy
– With fluctuating symptoms
– Using inconsistent T4 brands
– Taking interacting medications
However, low-risk patients on stable doses without changes may only need annual TSH monitoring if symptomatic monitoring suffices. Discuss an appropriate testing frequency with your provider.
Should you take levothyroxine if your TSH is normal?
In most cases, levothyroxine is not recommended if your TSH level is already within the normal reference range. However, there are a few exceptions:
– **Women trying to conceive**: Guidelines recommend TSH below 2.5 mIU/L for optimal fertility and fetal health. Levothyroxine may be started if TSH is 2.5-4.5 mIU/L.
– **Pregnant women**: TSH should be less than 2.5 mIU/L in the first trimester, or 3.0 mIU/L later in pregnancy. Levothyroxine is typically given if over these targets.
– **People with thyroid antibodies**: levothyroxine may be warranted in antibody-positive patients with recurrent miscarriage, infertility, or hypothyroid symptoms.
– **Individualized cases**: Some patients feel better with a lower TSH around 1.0-2.0 mIU/L. Levothyroxine may be appropriate based on symptoms.
– **Secondary hypothyroidism**: Since TSH levels are low in pituitary gland dysfunction, T4/T3 levels alone determine if treatment is needed.
Overall, levothyroxine is generally not advised if TSH is already normal in an otherwise healthy person without risk factors. Your doctor can advise if you are a candidate for preventative thyroid hormone therapy.
Takeaways
When taking levothyroxine, the optimal TSH level for most people falls between 0.5-2.5 mIU/L based on major endocrinology guidelines and clinical experience. However, the precise target can vary between individuals. Work with your doctor to determine the TSH level that relieves your hypothyroid symptoms and makes you feel your best on levothyroxine therapy. Recheck TSH levels every 6-12 months once stabilized to ensure the dose remains appropriate over time.
