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TSH (Thyroid-Stimulating Hormone)

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Thyroid-Stimulating Hormone (TSH)

TSH is a glycoprotein secreted by the basophilic cells of the anterior pituitary gland. Its secretion is regulated by the hypothalamic thyrotropin-releasing factor, somatostatin, biogenic amines, and thyroid hormones. TSH stimulates all stages of thyroid hormone synthesis and secretion, enhances lipolysis, and regulates metabolic activity.

Circadian Rhythm of TSH

TSH levels exhibit daily fluctuations:

  • Peak concentration occurs between 2–4 AM and in the morning.
  • Lowest concentration is observed between 6–8 AM and 5–6 PM.
  • Sleep disturbances or night awakenings can disrupt normal secretion rhythms.
  • During pregnancy, TSH levels tend to increase.
  • With aging, TSH levels gradually rise, while nocturnal secretion decreases.

Role of TSH

TSH regulates the secretion of thyroid hormones – thyroxine (T4) and triiodothyronine (T3).

  • A decrease in thyroid hormone levels triggers an increase in TSH, stimulating the thyroid gland to produce more hormones.
  • An increase in T4 and T3 levels leads to a reduction in TSH secretion.

Thyroid hormones are the primary regulators of energy metabolism, and maintaining their optimal levels is essential for proper organ function.

Pituitary Dysfunction and TSH Levels

  • Pituitary dysfunction can cause both an increase and a decrease in TSH levels.
  • Elevated TSH levels lead to excessive thyroid hormone production, potentially causing hyperthyroidism.
  • TSH secretion disorders may also result from hypothalamic dysfunction, due to excessive or deficient thyrotropin-releasing hormone (TRH) secretion.

TSH and Hypothyroidism

  • Primary hypothyroidism is characterized by elevated TSH levels and low Free T4, T4, and T3 levels.
  • Subclinical or mild hypothyroidism presents with TSH elevation while Free T4 and T4 remain within normal ranges.
  • Low TSH levels in hypothyroidism suggest a pituitary or hypothalamic origin, indicating secondary hypothyroidism rather than primary thyroid dysfunction.
     

    Reference Ranges for TSH by Age and Pregnancy Trimester

    Age Group

    TSH Level (mU/L)

    Newborns (1–4 days)

    1.1 – 17.0

    Preterm infants (28–36 weeks gestation)

    1.0 – 39.0

    Newborns (0.7–27 days)

    0.7 – 27.0

    < 2.5 months

    0.6 – 10.0

    2.5 months – 14 months

    0.4 – 7.0

    14 months – 5 years

    0.4 – 6.0

    5 – 14 years

    0.4 – 5.0

    > 14 years (adults)

    0.4 – 4.0

    Pregnancy Reference Ranges for TSH

    Pregnancy Trimester

    TSH Level (mU/L)

    First trimester

    0.1 – 2.5

    Second trimester

    0.2 – 3.0

    Third trimester

    0.3 – 3.0




     

    Monitoring and Follow-Up

  • TSH levels reflect changes in the pituitary-thyroid axis over the past 3–6 weeks.
  • Retesting is recommended 2 months after dose adjustments for medications that affect TSH levels.
  •  
  • It is recommended to take the test in the morning after 8–14 hours of fasting (water intake is allowed).
  • If taking the test in the afternoon, it should be done at least 4 hours after the last meal.
  • The test should be performed at the same time each visit to monitor hormonal fluctuations over time.
  • Steroid and thyroid hormone medications should be discontinued 48 hours before the test.
  • Excessive physical and emotional stress should be avoided for 24 hours before the test.
  • Smoking should be avoided for at least 3 hours before the test.
  • Detection of subclinical hypothyroidism
  • Routine monitoring of diagnosed hypothyroidism (1–2 times per year)
  • Follow-up testing for diffuse toxic goiter (1–3 times per month for 1.5–2 years)
  • Delayed mental and sexual development in children
  • Heart arrhythmias
  • Myopathy
  • Idiopathic hypothermia

     

Causes of Increased TSH Levels

  • Primary thyroid hypofunction
  • Endemic goiter
  • Thyroid inflammation (thyroiditis)
  • Post-treatment state after iodine therapy
  • Malignant thyroid neoplasms
  • Thyrotropinoma
  • Basophilic pituitary adenoma
  • Syndrome of uncontrolled TSH secretion
  • Thyroid hormone resistance syndrome
  • Primary and secondary hypothyroidism
  • Juvenile hypothyroidism
  • Uncompensated primary adrenal insufficiency
  • Hashimoto’s thyroiditis
  • Ectopic TSH secretion in lung neoplasms
  • Pituitary neoplasia
  • Severe somatic and psychiatric disorders
  • Severe preeclampsia
  • Cholecystectomy
  • Lead exposure
  • Excessive physical exertion
  • Hemodialysis

Medications that Increase TSH Levels

  • Anticonvulsants (valproic acid, phenytoin, benserazide)
  • Beta-blockers (atenolol, metoprolol, propranolol)
  • Amiodarone
  • Calcitonin
  • Antipsychotics (phenothiazine, aminoglutethimide)
  • Clomiphene
  • Motilium, metoclopramide
  • Iron sulfate
  • Furosemide
  • Iodides, X-ray contrast agents
  • Lovastatin
  • Methimazole
  • Morphine
  • Diphenin
  • Prednisolone
  • Rifampicin
     

    Causes of Decreased TSH Levels

  • Primary thyroid hyperfunction (hyperthyroidism)
  • Hypothalamic-pituitary insufficiency
  • Pituitary neoplasia
  • Pituitary trauma
  • Toxic goiter
  • Thyrotoxic adenoma
  • TSH-independent thyrotoxicosis
  • Pregnancy-related hyperthyroidism and postpartum pituitary necrosis
  • T3-toxicosis
  • Latent thyrotoxicosis
  • Transient thyrotoxicosis in autoimmune thyroiditis
  • Unsupervised intake of T4 (levothyroxine)
  • Pituitary damage
  • Psychological stress
  • Starvation, cachexia
  • Excessive thyroid hormone absorption
  • Cushing’s syndrome

  • Medications that Decrease TSH Levels

  • Acetylsalicylic acid (aspirin)
  • Heparin
  • Anabolic steroids
  • Corticosteroids
  • Cytostatics
  • Beta-adrenergic agonists (dobutamine, dopexamine)
  • Dopamine
  • Amiodarone (in hyperthyroid patients)
  • Thyroxine (T4), Triiodothyronine (T3)
  • Carbamazepine
  • Somatostatin, octreotide
  • Nifedipine
  • Drugs used to treat hyperprolactinemia (metergoline, peribedil, bromocriptine)