Because of the increased prevalence of cardiovascular disease among the elderly, initiate SYNTHROID at less than the full replacement dose [see Warnings and Precautions (5.1) and Dosage and Administration (2.3)]. Atrial arrhythmias can occur in elderly patients. Atrial fibrillation is the most common of the arrhythmias observed with levothyroxine overtreatment in the elderly.
The signs and symptoms of overdosage are those of hyperthyroidism [see Warnings and Precautions (5) and Adverse Reactions (6)]. In addition, confusion and disorientation may occur. Cerebral embolism, shock, coma, and death have been reported. Seizures occurred in a 3-year-old child ingesting 3.6 mg of levothyroxine. Symptoms may not necessarily be evident or may not appear until several days after ingestion of levothyroxine sodium.
SYNTHROID (levothyroxine sodium tablets, USP) contain synthetic crystalline L-3,3′,5,5′-tetraiodothyronine sodium salt [levothyroxine (T4) sodium]. Synthetic T4 is chemically identical to that produced in the human thyroid gland. Levothyroxine (T4) sodium has an empirical formula of C15 H10 I4 N NaO4 • H2 O, molecular weight of 798.86 (anhydrous), and structural formula as shown:
SYNTHROID tablets for oral administration are supplied in the following strengths: 25 mcg, 50 mcg, 75 mcg, 88 mcg, 100 mcg, 112 mcg, 125 mcg, 137 mcg, 150 mcg, 175 mcg, 200 mcg, and 300 mcg. Each SYNTHROID tablet contains the inactive ingredients acacia, confectioner’s sugar (contains corn starch), lactose monohydrate, magnesium stearate, povidone, and talc. Each tablet strength meets USP Dissolution Test 3. Table 6 provides a listing of the color additives by tablet strength:
|Table 6. SYNTHROID Tablets Color Additives|
|Strength (mcg)||Color additive(s)|
|25||FD&C Yellow No. 6 Aluminum Lakea|
|75||FD&C Red No. 40 Aluminum Lake, FD&C Blue No. 2 Aluminum Lake|
|88||FD&C Blue No. 1 Aluminum Lake, FD&C Yellow No. 6 Aluminum Lakea , D&C Yellow No. 10 Aluminum Lake|
|100||D&C Yellow No. 10 Aluminum Lake, FD&C Yellow No. 6 Aluminum Lakea|
|112||D&C Red No. 27 & 30 Aluminum Lake|
|125||FD&C Yellow No. 6 Aluminum Lakea , FD&C Red No. 40 Aluminum Lake, FD&C Blue No. 1 Aluminum Lake|
|137||FD&C Blue No. 1 Aluminum Lake|
|150||FD&C Blue No. 2 Aluminum Lake|
|175||FD&C Blue No. 1 Aluminum Lake, D&C Red No. 27 & 30 Aluminum Lake|
|200||FD&C Red No. 40 Aluminum Lake|
|300||D&C Yellow No. 10 Aluminum Lake, FD&C Yellow No. 6 Aluminum Lakea , FD&C Blue No. 1 Aluminum Lake|
Thyroid hormones exert their physiologic actions through control of DNA transcription and protein synthesis. Triiodothyronine (T3) and L-thyroxine (T4) diffuse into the cell nucleus and bind to thyroid receptor proteins attached to DNA. This hormone nuclear receptor complex activates gene transcription and synthesis of messenger RNA and cytoplasmic proteins.
Absorption of orally administered T4 from the gastrointestinal tract ranges from 40% to 80%. The majority of the SYNTHROID dose is absorbed from the jejunum and upper ileum. The relative bioavailability of SYNTHROID tablets, compared to an equal nominal dose of oral levothyroxine sodium solution, is approximately 93%. T4 absorption is increased by fasting, and decreased in malabsorption syndromes and by certain foods such as soybeans. Dietary fiber decreases bioavailability of T4. Absorption may also decrease with age. In addition, many drugs and foods affect T4 absorption [see Drug Interactions (7)].
Circulating thyroid hormones are greater than 99% bound to plasma proteins, including thyroxine-binding globulin (TBG), thyroxine-binding prealbumin (TBPA), and albumin (TBA), whose capacities and affinities vary for each hormone. The higher affinity of both TBG and TBPA for T4 partially explains the higher serum levels, slower metabolic clearance, and longer half-life of T4 compared to T3. Protein-bound thyroid hormones exist in reverse equilibrium with small amounts of free hormone. Only unbound hormone is metabolically active. Many drugs and physiologic conditions affect the binding of thyroid hormones to serum proteins [see Drug Interactions (7)]. Thyroid hormones do not readily cross the placental barrier [see Use in Specific Populations (8.1)].
T4 is slowly eliminated (see Table 7). The major pathway of thyroid hormone metabolism is through sequential deiodination. Approximately 80% of circulating T3 is derived from peripheral T4 by monodeiodination. The liver is the major site of degradation for both T4 and T3, with T4 deiodination also occurring at a number of additional sites, including the kidney and other tissues. Approximately 80% of the daily dose of T4 is deiodinated to yield equal amounts of T3 and reverse T3 (rT3). T3 and rT3 are further deiodinated to diiodothyronine. Thyroid hormones are also metabolized via conjugation with glucuronides and sulfates and excreted directly into the bile and gut where they undergo enterohepatic recirculation.
Thyroid hormones are primarily eliminated by the kidneys. A portion of the conjugated hormone reaches the colon unchanged and is eliminated in the feces. Approximately 20% of T4 is eliminated in the stool. Urinary excretion of T4 decreases with age.
|Table 7. Pharmacokinetic Parameters of Thyroid Hormones in Euthyroid Patients|
|Hormone||Ratio in Thyroglobulin||Biologic Potency||t1/2 (days)||Protein Binding (%)a|
|Levothyroxine (T4)||10 — 20||1||6-7b||99.96|
|Liothyronine (T3)||1||4||≤ 2||99.5|
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