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Synthroid - Scientific Information

Manufacture: Abbott
Country: Canada
Condition: Hashimoto's disease (Chronic Lymphocytic Thyroiditis), Hypothyroidism, After Thyroid Removal (Hypothyroidism, Post-Thyroidectomy), Myxedema Coma, Thyroid Suppression Test, TSH Suppression, Underactive Thyroid (Hypothyroidism)
Class: Thyroid drugs
Form: Tablets
Ingredients: levothyroxine sodium, acacia, confectioner's sugar, lactose, magnesium stearate, povidone, talc, colour additive

Pharmaceutical Information

Proper name: Levothyroxine sodium
Chemical name: L-3,3′,5,5′-tetraiodothyronine sodium salt
Molecular formula and molecular mass: C15H10I4N NaO4•H2O   798.86 g/mol (anhydrous)
Structural formula:

Physicochemical properties: Levothyroxine sodium occurs as a light yellow to buff-coloured, odourless, tasteless, hygroscopic powder. Levothyroxine sodium is very slightly soluble in water and slightly soluble in alcohol.

Clinical Trials

The published studies presented in this section support the effectiveness of SYNTHROID (levothyroxine sodium, USP) in the treatment of hypothyroidism. They are considered to have at least some of the characteristics of adequate and well-controlled as defined under ICH Good Clinical Practice. The controlled clinical studies are primarily: 1) studies that investigated the biochemical response to SYNTHROID of patients with hypothyroidism and the correlation of the optimal clinical dose with the pathology of hypothyroidism, 2) conventional studies of untreated hypothyroid patients or those switched from another brand of the same active drug, and 3) studies that analyze the dose-response characteristics in hypothyroid patients replaced with SYNTHROID or patients receiving SYNTHROID for suppression of TSH. In all cases, objective biochemical endpoints (e.g. TSH, T4, etc.), which minimize the potential for influence of chance or bias on results, were used to assess the effectiveness of SYNTHROID as replacement or suppressive therapy. The results of the studies demonstrate that with careful dose titration to an objective, biochemical endpoint, SYNTHROID is effective both for initial and maintenance therapy of hypothyroid adults. On the whole, the average L-thyroxine replacement doses reported in these studies are in close agreement with each other and average replacement doses reported in the literature and recommended by thyroid experts.

Study Demographics and Trial Design

Table 6. Summary of Patient Demographics for Clinical Trials in Specific Indication
Author / Manuscript Title Trial Design Dosage, Route of Administration and Duration Study Subjects (n=number) Mean Age (Range) Gender (M/F)
Kabadi UM., 1994/ “Optimal L-thyroxine dose in primary hypothyroidism”. Longitudinal 25-200 mcg/day
Oral dosage form
186 NR
(25-84 years)
Kabadi UM., 1989/ “Optimal L-thyroxine dose in hypothyroidism”. Longitudinal 50-200 mcg/day
Oral dosage form
156* NR
(25-84 years)
Kabadi UM, Jackson T., 1995/ “TSH predictor in hypothyroidism”. Longitudinal 25-225 mcg/day
Oral dosage form
192 NR
(25-84 years)
Hennessey J, et al., 1985/ “Equivalency of two L-thyroxine preparations”. Crossover 50-200 mcg/day
Oral dosage form
34 NR NR
Fish LH, et al., 1987/ “Replacement dose in hypothyroidism”. Longitudinal 25-150 mcg/day
Oral dosage form
19 NR NR
Ain KG, et al., 1996/ “Effects of restrictive formulary”. Longitudinal Restricted arm (n=87):
1.9 ± 0.1 mcg/kg/day
Non-restricted arm (n=148):
2.0 ± 0.1 mcg/kg/day
Oral dosage form
241 Restricted arm: (n=89):
39.3 ± 2.4 year
(range NR)
Non-restricted arm (n=152):
44.2 ± 1.3 year
(range NR)
Ain KG, et al., 1993/ “TFTs affected by time of blood sampling”. Longitudinal 150-200 mcg/day
Oral dosage form
51 NR NR
Sherman SI, et al., 1997/ “Effects of T3 compared to T4”. Longitudinal 2.2 – 2.6 mcg/kg/day (LT4-only arm)
Oral dosage form
(LT4-only arm)
Liu X-Q, et al., 1998/ “Effects of L-thyroxine on serum lipoproteins”. Longitudinal 183 (mean)
Oral dosage form
10 45.7 ±10.6 year
(range NR)
Hussein W, et al., 1999/ “Effects of L-thyroxine on serum homocysteine”. Longitudinal NR 14 45.7
(25 – 77 yr)

* This is considered to be an earlier publication of the same patient population presented in Kabadi, 1994. The 156 patients described are not added into the total number of patients.

NR = not reported

LT4 = L-thyroxine