Thyroid Laboratory Tests*
GeneralThe diagnosis of hypothyroidism is confirmed by measuring TSH levels using a sensitive assay (second generation assay sensitivity
0.1 mIU/L or third generation assay sensitivity
0.01 mIU/L) and measurement of free-T4.
The adequacy of therapy is determined by periodic assessment of appropriate laboratory tests and clinical evaluation. 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 (see PRECAUTIONS, Drug Interactions and Drug-Laboratory Test Interactions). Persistent clinical and laboratory evidence of hypothyroidism despite an apparent adequate replacement dose of LEVOXYL® may be evidence of inadequate absorption, poor compliance, drug interactions, or decreased T4 potency of the drug product.
Adults
In adult patients with primary (thyroidal) hypothyroidism, serum TSH
levels (using a sensitive assay) alone may be used to monitor therapy.
The frequency of TSH monitoring during levothyroxine dose titration
depends on the clinical situation but it is generally recommended at
6–8 week intervals until normalization. For patients who have recently
initiated levothyroxine therapy and whose serum TSH has normalized or
in patients who have had their dosage or brand of levothyroxine changed,
the serum TSH concentration should be measured after 8–12 weeks.
When the optimum replacement dose has been attained, clinical (physical
examination) and biochemical monitoring may be performed every 6–12
months, depending on the clinical situation, and whenever there is a
change in the patient’s status. It is recommended that a physical
examination and a serum TSH measurement be performed at least annually
in patients receiving LEVOXYL® (see WARNINGS,
PRECAUTIONS, and DOSAGE AND ADMINISTRATION).
Pediatrics
In patients with congenital hypothyroidism, the adequacy of replacement
therapy should be assessed by measuring both serum TSH (using a sensitive
assay) and total- or free-T4. During the first three years
of life, the serum total- or free-T4 should be maintained
at all times in the upper half of the normal range. While the aim of
therapy is to also normalize the serum TSH level, this is not always
possible in a small percentage of patients, particularly in the first
few months of therapy. TSH may not normalize due to a resetting of the
pituitary-thyroid feedback threshold as a result of in utero hypothyroidism.
Failure of the serum T4 to increase into the upper half of
the normal range within 2 weeks of initiation of LEVOXYL®
therapy and/or of the serum TSH to decrease below 20 mU/L within 4 weeks
should alert the physician to the possibility that the child is not
receiving adequate therapy. Careful inquiry should then be made regarding
compliance, dose of medication administered, and method of administration
prior to raising the dose of LEVOXYL®.
The recommended frequency of monitoring of TSH and total or free T4 in children is as follows: at 2 and 4 weeks after the initiation of treatment; every 1–2 months during the first year of life; every 2–3 months between 1 and 3 years of age; and every 3 to 12 months thereafter until growth is completed. More frequent intervals of monitoring may be necessary if poor compliance is suspected or abnormal values are obtained. It is recommended that TSH and T4 levels, and a physical examination, if indicated, be performed 2 weeks after any change in Levoxyl® dosage. Routine clinical examination, including assessment of mental and physical growth and development, and bone maturation, should be performed at regular intervals (see PRECAUTIONS, Pediatric Use and DOSAGE AND ADMINISTRATION).
Secondary (pituitary) and tertiary (hypothalamic) hypothyroidism
Adequacy of therapy should be assessed by measuring serum free-T4
levels, which should be maintained in the upper half of the normal range
in these patients.
*This information is an excerpt from the full prescribing information.
