Dosage & Administration8
The dosage of thyroid hormones is determined by the indication and must be individualized according to patient response and laboratory findings. Thyroid hormones are given orally. Intramuscular administration is not advisable because of reported poor absorption.
Therapy is usually instituted using low doses, with increments which depend on the cardiovascular status of the patient. The usual starting dose is 30 mg NP Thyroid®, with increments of 15 mg every 2 to 3 weeks. A lower starting dosage, 15 mg/day, is recommended in patients with long standing myxedema, particularly if cardiovascular impairment is suspected, in which case extreme caution is recommended. The appearance of angina is an indication for a reduction in dosage. Most patients require 60 to 120 mg/day. Failure to respond to doses of 180 mg suggests lack of compliance or malabsorption. Maintenance dosages 60 to 120 mg/day usually result in normal serum levothyroxine (T4) and liothyronine (T3) levels. Adequate therapy usually results in normal TSH and T4 levels after four weeks of therapy after proper clinical and laboratory evaluations, including serum levels of T4 (bound and free) and TSH. Readjustment of thyroid hormone dosage should be made within the first four weeks of therapy, after proper clinical and laboratory evaluations, including serum levels of T4, bound and free, and TSH. T3 may be used in preference to levothyroxine (T4) during radio-isotope scanning procedures, since induction of hypothyroidism in those cases is more abrupt and can be of shorter duration. It may also be preferred when impairment of peripheral conversion of T4 and T3 is suspected.
Myxedema coma is usually precipitated in the hypothyroid patient of long- standing by intercurrent illness or drugs such as sedatives and anesthetics and should be considered a medical emergency. Therapy should be directed at the correction of electrolyte disturbances and possible infection besides the administration of thyroid hormones. Corticosteroids should be administered routinely. T4 and T3 may be administered via a nasogastric tube but the preferred route of administration of both hormones is intravenous. Levothyroxine sodium (T4) is given at starting dose of 400 mcg (100 mcg/mL) given rapidly, and is usually well tolerated, even in the elderly. This initial dose is followed by daily supplements of 100 to 200 mcg given intravenously. Normal T4 levels are achieved in 24 hours followed in 3 days by threefold elevation of T3. Oral therapy with thyroid hormone would be resumed as soon as the clinical situation has been stabilized and the patient is able to take oral medication.
Exogenous thyroid hormone may produce regression of metastases from follicular and papillary carcinoma of the thyroid and is used as ancillary therapy of these conditions with radio-active iodine. TSH should be suppressed to low or undetectable levels. Therefore, larger amounts of thyroid hormone than those used for replacement therapy are required. Medullary carcinoma of the thyroid is usually unresponsive to this therapy.
Administration of thyroid hormone in doses higher than those produced physiologically by the gland results in suppression of the production of endogenous hormone. This is the basis for the thyroid suppression test and is used as an aid in the diagnosis of patients with signs of mild hyperthyroidism in whom base line laboratory tests appear normal, or to demonstrate thyroid gland autonomy in patients with Grave’s ophthalmopathy. 131I uptake is determined before and after the administration of the exogenous hormone. A 50 percent or greater suppression of uptake indicates a normal thyroid-pituitary axis and thus rules out thyroid gland autonomy. For adults, the usual suppressive dose of levothyroxine (T4) is 1.56 mcg/kg of body weight per day given for 7 to 10 days. These doses usually yield normal serum T4 and T3 levels and lack of response to TSH. Thyroid hormones should be administered cautiously to patients in whom there is strong suspicion of thyroid gland autonomy, in view of the fact that the exogenous hormone effects will be additive to the endogenous source.
Pediatric dosage should follow the recommendations summarized in the table below. In infants with congenital hypothyroidism, therapy with full doses should be instituted as soon as the diagnosis has been made.
|Recommended Pediatric Dosage of NP Thyroid Tablets for Congenital Hypothyroidism8|
|Age||Dose per day||Daily dose per kg of body weight|
|0-6 months||15 – 30 mg||4.8 – 6 mg|
|6-12 months||30 – 45 mg||3.8 – 4.8 mg|
|1-5 years||45 – 60 mg||3 – 3.6 mg|
|6-12 years||60 – 90 mg||2.4 – 3 mg|
|Over 12 years||Over 90 mg||1.2 – 1.8 mg|