Levothyroxine is Used to Treat High TSH Levels

Levothyroxine is prescribed for elevated TSH levels, which indicate hypothyroidism—a state of insufficient thyroid hormone production. 1, 2, 3, 4

Understanding the TSH-Thyroid Hormone Relationship

High TSH signals hypothyroidism: When the thyroid gland fails to produce adequate thyroid hormone (T4 and T3), the pituitary gland compensates by releasing more TSH in an attempt to stimulate the underperforming thyroid gland 1, 4
Low TSH signals hyperthyroidism or overtreatment: Conversely, when thyroid hormone levels are excessive (either from an overactive thyroid or too much levothyroxine medication), TSH becomes suppressed as the pituitary detects adequate or excessive thyroid hormone 1
TSH acts as the body's thermostat: The pituitary uses TSH to regulate thyroid hormone production—increasing TSH when more thyroid hormone is needed, and decreasing TSH when thyroid hormone levels are sufficient 1, 4

For overt hypothyroidism (high TSH with low free T4):

Start levothyroxine immediately when TSH is elevated AND free T4 is below the reference range, regardless of symptoms 1, 4
This represents overt hypothyroidism requiring prompt treatment to prevent cardiovascular dysfunction, adverse lipid profiles, and quality of life deterioration 1

For subclinical hypothyroidism (high TSH with normal free T4):

TSH >10 mIU/L: Initiate levothyroxine therapy regardless of symptoms, as this level carries approximately 5% annual risk of progression to overt hypothyroidism 1, 5, 4
TSH 4.5-10 mIU/L: Treatment decisions should be individualized based on:
- Presence of hypothyroid symptoms (fatigue, weight gain, cold intolerance, constipation) 1, 6
- Positive anti-TPO antibodies (indicating 4.3% annual progression risk vs 2.6% in antibody-negative patients) 1, 5
- Pregnancy or planning pregnancy (subclinical hypothyroidism associated with adverse pregnancy outcomes) 1, 5
- Age under 65 years (treatment may be harmful in elderly patients with mild TSH elevation) 7
Confirm diagnosis: Repeat TSH testing after 3-6 weeks before initiating treatment, as 30-60% of elevated TSH levels normalize spontaneously 1, 6, 7

Initial dosing for adults:

Younger patients (<70 years) without cardiac disease: Start with full replacement dose of approximately 1.6 mcg/kg/day 1, 2, 3, 5
Elderly patients (>70 years) or those with cardiac disease: Start with lower dose of 25-50 mcg/day and titrate gradually to avoid cardiac complications 1, 2, 3, 5
Pregnant patients with new-onset hypothyroidism: 1.6 mcg/kg/day if TSH ≥10 IU/L, or 1.0 mcg/kg/day if TSH <10 IU/L 3

Critical safety consideration:

In patients with suspected central hypothyroidism or concurrent adrenal insufficiency, always start corticosteroids before levothyroxine to prevent life-threatening adrenal crisis 1, 5

Check TSH every 6-8 weeks after initiating treatment or changing dose, as levothyroxine has a long half-life requiring this interval to reach steady state 1, 2, 3, 4
Target TSH range: 0.5-4.5 mIU/L for primary hypothyroidism in most patients 1, 5
Once stable: Monitor TSH every 6-12 months or when clinical status changes 1, 3, 4
Dose adjustments: Increase or decrease by 12.5-25 mcg increments based on TSH response 1

Overtreatment occurs in 14-25% of patients: Approximately 25% of patients on levothyroxine are unintentionally maintained on doses high enough to suppress TSH completely, increasing risks for atrial fibrillation, osteoporosis, fractures, and cardiac complications 1, 2, 5, 4
Don't treat based on single elevated TSH: 30-60% of elevated TSH levels normalize on repeat testing, representing transient thyroiditis or laboratory variation 1, 6, 7
Recognize transient hypothyroidism: Some causes of hypothyroidism (subacute thyroiditis, postpartum thyroiditis, medication-induced) may resolve spontaneously and not require lifelong treatment 1, 6
Age-adjusted TSH targets: The upper limit of normal TSH increases with age (97.5th percentile is 3.6 mIU/L for patients under 40, but 7.5 mIU/L for patients over 80) 7