12.1 Mechanism of Action
Thyroid hormones exert their physiologic actions through control of DNA transcription and protein synthesis. Triiodothyronine (T
3) and levothyroxine (T
4) 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.
The physiological actions of thyroid hormones are produced predominantly by T
3, the majority of which (approximately 80%) is derived from T
4 by deiodination in peripheral tissues.
12.2 Pharmacodynamics
Thyroid hormone synthesis and secretion is regulated by the hypothalamic pituitary-thyroid axis. Thyrotropin releasing hormone (TRH) released from the hypothalamus stimulates secretion of thyrotropin stimulating hormone (TSH) from the anterior pituitary. TSH, in turn, is the physiologic stimulus for the synthesis and secretion of thyroid hormones, T
4 and T
3, by the thyroid gland. Circulating serum T
3 and T
4 levels exert a feedback effect on both TRH and TSH secretion. When serum T
3 and T
4 levels increase, TRH and TSH secretion decrease. When thyroid hormone levels decrease, TRH and TSH secretion increases. TSH is used for the diagnosis of hypothyroidism and evaluation of levothyroxine therapy adequacy with other laboratory and clinical data.
[See Dosage (
2.1)]
There are drugs known to affect thyroid hormones and TSH by various mechanisms and those examples are diazepam, ethioamide, lovastatin, metoclopramide, 6-mercaptopurine, nitroprusside, perphenazine, and thiazide diuretics. Some drugs may cause a transient decrease in TSH secretion without hypothyroidism and those drugs (dose) are dopamine (greater than 1 mcg per kg per min), glucocorticoids (hydrocortisone greater than 100 mg per day or equivalent) and octreotide (greater than 100 mcg per day).
Thyroid hormones regulate multiple metabolic processes and play an essential role in normal growth and development, and normal maturation of the central nervous system and bone. The metabolic actions of thyroid hormones include augmentation of cellular respiration and thermogenesis, as well as metabolism of proteins, carbohydrates and lipids. The protein anabolic effects of thyroid hormones are essential to normal growth and development.
12.3 Pharmacokinetics
Absorption – Levothyroxine Sodium for Injection is administered via the intravenous route. Following administration, the synthetic levothyroxine cannot be distinguished from the natural hormone that is secreted endogenously.
Distribution – 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 T
4 partially explains the higher serum levels, slower metabolic clearance, and longer half-life of T
4 compared to T
3. 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 Warnings and Precautions (
5) and
Drug Interactions (
7)]. Thyroid hormones do not readily cross the placental barrier [
See Warnings and Precautions (
5) and
Use in Specific Populations (
8)].
Metabolism – T
4 is slowly eliminated. The major pathway of thyroid hormone metabolism is through sequential deiodination. Approximately eighty percent of circulating T
3 is derived from peripheral T
4 by monodeiodination. The liver is the major site of degradation for both T
4 and T
3, with T
4 deiodination also occurring at a number of additional sites, including the kidney and other tissues. Approximately 80% of the daily dose of T
4 is deiodinated to yield equal amounts of T
3 and reverse T
3 (r T
3). T
3 and r T
3 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.
Elimination – Thyroid hormones are primarily eliminated by the kidneys. A portion of the conjugated hormone reaches the colon unchanged, where it is hydrolyzed and eliminated in feces as the free hormones. Urinary excretion of T
4 decreases with age.
Table 1: Pharmacokinetic Parameters of Thyroid Hormones in Euthyroid Patients
Hormone | Ratio in Thyroglobulin | Biologic Potency | Half-Life (Days) | Protein Binding (%)
2
|
T
4
| 10 - 20 | 1 | 6 - 8
1
| 99.96 |
T
3
| 1 | 4 | ≤ 2 | 99.5 |
T
4: Levothyroxine
T
3: Liothyronine
1 3 to 4 days in hyperthyroidism, 9 to 10 days in hypothyroidism
2 Includes TBG, TBPA, and TBA.
Drug Interactions
A listing of drug interaction with T
4 is provided in the following tables, although it may not be comprehensive due to the introduction of new drugs that interact with the thyroidal axis or the discovery of previously unknown interactions. The prescriber should be aware of this fact and should consult appropriate reference sources (e.g., package inserts of newly approved drugs, medical literature) for additional information if a drug-drug interaction with levothyroxine is suspected.
Table 2: Drugs That May Alter T
4 and T
3 Serum Transport Without Affecting free T
4 Concentration (Euthyroidism)
Drugs That May Increase Serum TBG Concentration | Drugs That May Decrease Serum TBG Concentration |
Clofibrate Estrogen-containing oral contraceptives Estrogens (oral) Heroin/ Methadone 5-Fluorouracil Mitotane Tamoxifen | Androgens / Anabolic Steroids Asparaginase Glucocorticoids Slow-Release Nicotinic Acid |
Drugs That May Cause Protein-Binding Site Displacement Potential impact: Administration of these agents with levothyroxine results in an initial transient increase in FT
4. Continued administration results in a decrease in serum T
4 and normal FT
4 and TSH concentrations and, therefore, patients are clinically euthyroid.
|
Salicylates (> 2g/day) | Salicylates inhibit binding of T
4 and T
3 to TBG and transthyretin. An initial increase in serum FT
4 is followed by return of FT
4 to normal levels with sustained therapeutic serum salicylate concentrations, although total-T
4 levels may decrease by as much as 30%.
|
Other drugs: Furosemide (>80 mg IV) Heparin Hydantoins Non-Steroidal Anti-inflammatory Drugs - Fenamates - Phenylbutazone | |
Table 3: Drugs That May Alter Hepatic Metabolism of T
4 (Hypothyroidism)
Potential impact: Stimulation of hepatic microsomal drug-metabolizing enzyme activity may cause increased hepatic degradation of levothyroxine, resulting in increase levothyroxine requirements.
Drug or Drug Class | |
Carbamazepine Hydantoins | Phenytoin and carbamazepine reduce serum protein binding of levothyroxine, and total- and free-T
4 may be reduced by 20% to 40%, but most patients have normal serum TSH levels and are clinically euthyroid.
|
Other drugs: Phenobarbital Rifampin | |
Table 4: Drugs That May Decrease Conversion of T
4 to T
3
Potential impact: Administration of these enzyme inhibitors decreases the peripheral conversion of T
4 to T
3, leading to decreased T
3 levels. However, serum T4 levels are usually normal but may occasionally be slightly increased.
Drug or Drug Class | Effect |
Beta-adrenergic antagonists (e.g. Propranolol > 160 mg/day) | In patients treated with large doses of propranolol (> 160 mg/day), T3 and T4 levels change slightly, TSH levels remain normal, and patients are clinically euthyroid. It should be noted that actions of particular beta-adrenergic antagonists may be impaired when the hypothyroid patient is converted to the euthyroid state. |
Glucocorticoids (e.g. Dexamethasone ≥ 4 mg/day) | Short-term administration of large doses of glucocorticoids may decrease serum T
3 concentrations by 30% with minimal change in serum T
4 levels. However, long-term glucocorticoid therapy may result in slightly decreased T
3 and T
4 levels due to decreased TBG production (See above).
|
Other drug: Amiodarone | |