TSH Changes With Adjustments
of Thyroxine Dosage

Carr Study

Adapted from a 1988 biochemical study conducted in 21 adult Caucasian patients in the United Kingdom with primary hypothyroidism who were tested on a series of different thyroxine dosages to evaluate effectiveness of measuring TSH levels to monitor thyroid function.

Following a 25 mcg Dose Change of Levothyroxine,
Most Patients Had Changes in TSH Levels6

Changes in TSH levels with adjustments of thyroxine dosage graph, based on Carr Study

The “optimal dose” was determined for each patient as that dosage of thyroxine being taken when the TRH response was normal (i.e., an increase in TSH between 4.7 and 25 mIU/L).

  • 10 patients were dosed 25 mcg higher than their optimal dose (the dose at which each patient had a normal response to thyrotropin-releasing hormone)
  • 2 patients were dosed 25 mcg lower than their optimal dose
  • 9 patients were dosed 25 mcg higher and lower than their optimal dose

Biochemical assessment incorporated measurement of serum TSH, T3, and T4. TSH lower limit of quantification was 0.2 mIU/L and upper limit of normal was 5.6 mIU/L, as indicated by the shaded area. Each dot represents one patient.

Indication and Important Safety Information for Synthroid


SYNTHROID® (levothyroxine sodium) tablets, for oral use is indicated as a replacement therapy in primary (thyroidal), secondary (pituitary), and tertiary (hypothalamic) congenital or acquired hypothyroidism.

Pituitary Thyrotropin (Thyroid Stimulating Hormone, TSH) Suppression
SYNTHROID is indicated as an adjunct to surgery and radioiodine therapy in the management of thyrotropin-dependent well-differentiated thyroid cancer.

Limitation of Use1

SYNTHROID is not indicated for suppression of benign thyroid nodules and nontoxic diffuse goiter in iodine-sufficient patients, as there are no clinical benefits and over-treatment with SYNTHROID may induce hyperthyroidism.

SYNTHROID is not indicated for treatment of hypothyroidism during the recovery phase of subacute thyroiditis.

Important Safety Information1


Thyroid hormones, including SYNTHROID, either alone or with other therapeutic agents, should not be used for the treatment of obesity or for weight loss. In euthyroid patients, doses within the range of daily hormonal requirements are ineffective for weight reduction. Larger doses may produce serious or even life-threatening manifestations of toxicity, particularly when given in association with sympathomimetic amines such as those used for their anorectic effects.

  • SYNTHROID is contraindicated in patients with uncorrected adrenal insufficiency.
  • In the elderly and in patients with cardiovascular disease, SYNTHROID should be initiated at lower doses than those recommended in younger individuals or in patients without cardiac disease. If cardiac symptoms develop or worsen, the SYNTHROID dose should be reduced or withheld for one week and restarted at a lower dose.
  • Patients with coronary artery disease who are receiving SYNTHROID should be monitored closely during surgical procedures for cardiac arrhythmias. Monitor patients during concomitant administration of SYNTHROID and sympathomimetic agents for signs and symptoms of coronary insufficiency.
  • Use of oral thyroid hormone is not recommended in myxedema coma. Products formulated for IV administration should be used to treat myxedema coma.
  • Patients with adrenal insufficiency should be treated with replacement glucocorticoids prior to initiating treatment with SYNTHROID. Failure to do so may precipitate an acute adrenal crisis when thyroid hormone therapy is initiated.
  • SYNTHROID has a narrow therapeutic index. Regardless of the indication for use, careful dosage titration is necessary to avoid the consequences of over- or under-treatment.
  • Addition of levothyroxine therapy in patients with diabetes mellitus may worsen glycemic control and result in increased antidiabetic agent or insulin requirements. Carefully monitor glycemic control after starting, changing, or discontinuing SYNTHROID.
  • Increased bone resorption and decreased bone mineral density may occur as a result of levothyroxine over-replacement, particularly in postmenopausal women. To mitigate this risk, patients receiving SYNTHROID should be given the minimum dose necessary that achieves the desired response.  
  • Adverse reactions associated with SYNTHROID therapy are primarily those of hyperthyroidism due to therapeutic overdosage.
  • Many drugs and some foods affect thyroid hormone pharmacokinetics and metabolism and may alter the therapeutic response to SYNTHROID. In addition, thyroid hormones and thyroid status have varied effects on the pharmacokinetics and actions of other drugs. Administer at least 4 hours before or after drugs that are known to interfere with absorption. Evaluate the need for dose adjustments when regularly administering within one hour of certain foods that may affect absorption. Prescribers should consult appropriate reference sources for additional information on drug or food interactions with SYNTHROID.
  • SYNTHROID should not be discontinued during pregnancy, and hypothyroidism diagnosed during pregnancy should be promptly treated. TSH levels may increase during pregnancy, so TSH should be monitored and SYNTHROID dose adjusted as needed.