What can cause fluctuations in Thyroid-Stimulating Hormone (TSH) levels?

From the Guidelines

Thyroid-Stimulating Hormone (TSH) levels can fluctuate due to numerous factors, including medication use, illness, and lifestyle changes, with common causes of fluctuations including levothyroxine, amiodarone, lithium, and certain steroids, as well as timing of blood tests, acute illnesses, pregnancy, dietary factors, stress, seasonal variations, and laboratory errors. When considering the factors that influence TSH levels, it's essential to recognize the role of medication, particularly levothyroxine, which is a synthetic thyroid hormone, as well as other medications like amiodarone, lithium, and certain steroids, which can affect TSH levels 1. The timing of blood tests is also crucial, given that TSH has a natural daily rhythm, with higher levels typically occurring at night, and acute illnesses, such as severe infections or hospitalizations, can temporarily suppress TSH levels through what's called "non-thyroidal illness syndrome" 1. Additionally, pregnancy dramatically alters thyroid function, with TSH often decreasing in the first trimester, and dietary factors like iodine intake extremes (either deficiency or excess) and supplements containing biotin can interfere with test results 1. Stress, both physical and emotional, can impact TSH through effects on the hypothalamic-pituitary axis, and seasonal variations have been documented, with TSH typically higher in colder months, while laboratory errors or inconsistent testing methods between different facilities can also create apparent fluctuations 1. Understanding these factors is vital to explain why occasional TSH variations may not necessarily indicate thyroid disease and why repeated testing is often necessary for accurate diagnosis. Some key points to consider when evaluating TSH fluctuations include:

• Medication use, such as levothyroxine, amiodarone, lithium, and certain steroids
• Timing of blood tests and the natural daily rhythm of TSH
• Acute illnesses and their potential to suppress TSH levels
• Pregnancy and its effects on thyroid function
• Dietary factors, including iodine intake and biotin supplements
• Stress and its impact on the hypothalamic-pituitary axis
• Seasonal variations in TSH levels
• Laboratory errors and inconsistencies in testing methods.

From the FDA Drug Label

Agents such as iron and calcium supplements and antacids can decrease the absorption of levothyroxine sodium tablets. Many drugs affect thyroid hormone pharmacokinetics and metabolism (e.g., absorption, synthesis, secretion, catabolism, protein binding, and target tissue response) and may alter the therapeutic response to Levothyroxine Sodium Tablets, USP. The choice of laboratory tests depends on various factors including the etiology of the underlying thyroid disease, the presence of concomitant medical conditions, including pregnancy, and the use of concomitant medications

The following can cause fluctuations in TSH levels:

• Medications: Many drugs can affect thyroid hormone pharmacokinetics and metabolism, altering the therapeutic response to levothyroxine sodium tablets, USP 2.
• Concomitant medical conditions: Presence of concomitant medical conditions, including pregnancy, can affect TSH levels 2.
• Use of concomitant medications: Use of concomitant medications can affect TSH levels 2.
• Agents that decrease absorption: Agents such as iron and calcium supplements and antacids can decrease the absorption of levothyroxine sodium tablets, potentially causing fluctuations in TSH levels 2.

From the Research

Causes of Fluctuations in TSH Levels

• Age: TSH levels increase with age 3
• Transient hypothyroidism: certain situations can lead to transient hypothyroidism that may last several weeks or months, causing fluctuations in TSH levels 3
• Medications: certain drugs, such as iron and calcium, can reduce the gastrointestinal absorption of levothyroxine, while enzyme inducers can reduce its efficacy 3
• Other medications: glucocorticoids, dopamine agonists, somatostatin analogues, and rexinoids can affect thyroid function through suppression of TSH in the thyrotrope or hypothalamus 4
• Autoimmune diseases: conditions such as chronic autoimmune thyroiditis can cause hypothyroidism and fluctuations in TSH levels 3, 5
• Radiation therapy: radiation therapy can damage the thyroid gland, leading to hypothyroidism and fluctuations in TSH levels 5
• Thyroid gland removal surgeries: surgical removal of the thyroid gland can lead to hypothyroidism and fluctuations in TSH levels 5
• Levothyroxine therapy: changes in levothyroxine dose or brand can cause fluctuations in TSH levels 6, 7
• Pregnancy: pregnancy can affect thyroid hormone levels and cause fluctuations in TSH levels 6
• Other medications: certain medications can interfere with the absorption or metabolism of levothyroxine, leading to fluctuations in TSH levels 6

Factors Influencing TSH Level Variations

• Spontaneous TSH level variations: athyreotic patients with differentiated thyroid cancer can experience spontaneous TSH level variations, leading to dose adjustments 7
• Age-specific TSH range: dose adjustments are often made in relation to the age-specific TSH range 7
• Levothyroxine dose: changes in levothyroxine dose can cause fluctuations in TSH levels 7