Synthroid is a brand name for the drug levothyroxine, which is prescribed to substitute a hormone normally produced by the thyroid gland to regulate energy and metabolism. Synthroid helps to replace and provide additional thyroid hormone in the body.
Low thyroid levels can happen naturally when the thyroid glands are affected by radiation, medication, or removed by surgery.
Adequate thyroid hormone levels are essential to maintain the normal mental and physical activity of a person. In children, adequate thyroid hormone is needed for the child’s normal mental and physical development.
When the thyroid hormone of the person is insufficient, the body of the person cannot function properly which may result in poor growth, slow speech, lack of energy, excessive tiredness, constipation, weight gain, hair loss, dry and thick skin, increased sensitivity to cold, muscle and joint pain.
Some people may not be able to take Synthroid, such as patients with current symptoms of heart attack and an untreated or uncontrolled disorder of the adrenal gland, a thyroid disorder called thyrotoxicosis.
Thyrotoxicosis is a state that happens when a person has too much thyroid hormone in the body. Synthroid is a thyroid hormone drug that should not be used for obesity and weight loss. This medication should not also be used to treat infertility unless the infertility is caused by a low thyroid hormone level.
To ensure that Synthroid is appropriate for the patient, assess the patient for:
- a thyroid nodule
- heart disease, blood clot, and disorder in blood clotting
- diabetes
- kidney disease
- anemia or lack of red blood cells
- osteoporosis or low bone mineral density
- problems with the pituitary gland
- any food or drug allergies
- any recently received radiation therapy which includes iodine
- breastfeeding status
Indications of Synthroid
Synthroid is indicated for the replacement therapy for primary(thyroidal), secondary(pituitary), and tertiary (hypothalamic) hypothyroidism. Synthroid is used to treat:
- Hypothyroidism. Hypothyroidism is a state where the person’s thyroid does not generate and release adequate thyroid hormone into the person’s bloodstream.
- Enlarged thyroid gland(goiter). Goiter is the abnormal enlargement of the person’s thyroid gland.
Synthroid is also used for patients that need to undergo surgery and radioiodine therapy that manages the type of cancer called well-differentiated thyroid cancer.
Mechanism of Action of Synthroid
Thyroid hormones use their physiologic actions by controlling the body’s DNA transcription and the synthesis of protein. The body’s Triiodothyronine(T3) and L- thyroxine (T4) are diffused to the cell nucleus and bin to the body’s thyroid receptor protein that is attached to the DNA.
This hormone nuclear receptor complex stimulates the person’s gene transcription and synthesis of messenger RNA and the cytoplasmic proteins.
The physiological actions of the thyroid hormones are created predominantly by the T3, the majority is derived from the body’s T4 by deiodination in the body’s peripheral tissue.
Thyroid hormones increase the metabolic rate of the body’s cells and tissues. The thyroid hormones have been revealed to exert both genomic and nongenomic effects. They exert the genomic effects by diffusing into the body cell nucleus and binding the thyroid hormone receptors in DNA regions called the thyroid hormone response element(TREs) near genes.
The complex of T4, T3, DNA, and the other coregulatory proteins will cause conformational change and will result in a shift in the transcriptional regulation of the nearby genes, synthesis of messenger RNA, and the production of the cytoplasmic protein.
The non-genomic action of the thyroid hormones to the body has been shown to happen when the binding to a plasma membrane receptor integrin aVb3 at the Arg-Gly-Asp site of recognition. From the surface cell, T4 binds to integrin resulting in a downstream effect that includes activation of mitogen-activated protein kinase that causes subsequent effects on cellular and nuclear events including angiogenesis and tumor cell proliferation.
Pharmacokinetics of Synthroid
- Absorption. Absorption of orally administered T4 from the person’s gastrointestinal tract ranges from 40 to 80 percent. Synthroid majority dose is absorbed from the body’s jejunum and upper ileum. The bioavailability of Synthroid tablets, compared to the nominal dose of oral levothyroxine sodium solution is at a percentage of 93%. The absorption of T4 is increased through fasting and decreased in malabsorption syndromes and by certain foods such as soybeans. The dietary fibers decrease the bioavailability of the body’s T4. The absorption of Synthroid appears to be influenced by the gastric pH of the body. The euthyroid patients who are suffering from non-toxic multinodular goiter, impaired gastric acid secretion, or the use of certain medication were associated with the increased dosing requirements to suppress TSH.
- Distribution. Thyroid hormones that circulate in the person’s body are greater than 99% bound to the plasma proteins that include thyroxine-binding globulin (TBG), thyroxine-binding prealbumin(TBPA), and albumin(TBA) vary from each hormone. Increases affinity of both TBG and TBPA for T4 will partially explain the increased serum levels, slower metabolic clearance, and longer half-life of the body’s T4. Protein-bound thyroid hormone happens in reverse equilibrium with the small amounts of the free hormone. The unbound hormone is the only metabolically active. The binding of the thyroid hormones and the serum proteins may be affected by the drugs and physiologic conditions. Synthroid has a limited volume of distribution that has been described to be 11.6 liters in euthyroid volunteers and 14.7 liters in the primary hypothyroid subjects. This is the approximate equivalent of the extracellular fluid in the person’s body.
- Metabolism. T4 is subjected to multiple metabolic reactions which involve the metabolic route for t4 involves deiodination reaction or removal of the iodine by the deiodinase enzymes. Removal of iodine from the carbon 5 of the outer ring transforms T4 and the T3, thus the T4 can be regarded somewhat as a pro hormone for the T3. Deiodination of the inner ring of t4 can also occur leading to the formation of inactive reverse T3.
- Elimination. Sequential deiodination is the major pathway for the metabolism of the thyroid hormone drug Synthroid. The circulating T3 is derived from the peripheral T4 through mono deiodination. The degradation of T4 and T3 with the T4 deiodination occurs at numerous additional sites including the kidney and other tissues. The daily turnover rate for T4 is about 10% while it is about 50-70% for T3, with a slightly faster turnover rate compared to patients with primary hypothyroidism. This is equivalent to a half-life for T4 of 7.5 days in hypothyroid patients and 6.2 days in euthyroid individuals while the T3 half-life is about 1.4 to 1.0 days for hypothyroid and euthyroid volunteers.
- Excretion. The thyroid hormone drug Synthroid is eliminated primarily by the person’s kidneys. The part of the conjugated hormone reaches the colon unchanged and is eliminated in the person’s feces.
Side Effects of Synthroid
Synthroid has dangerous side effects or death may occur because of the misuse of the medication, especially if the patient is taking any other weight-loss medication or other appetite suppressants.
Mild side effects of Synthroid include:
- Weight loss
- Hair loss
- Rash
- Headache
- Digestive problems
- Heat intolerance
- Fever
- Excessive sweating
- Spasm or weakness of muscles
- Shortness of breath
- Anxiety and irritability
- Irregular menstrual periods
- Difficulty in sleeping
These side effects are temporary and will last a few days or weeks.
Other side effects of Synthroid may include:
- Fast and irregular heartbeats
- Chest pain, pain that radiates to the jaw or shoulder
- Shortness of breath
- Fever, hot flashes, and sweating
- Tremors, and feeling of unusual cold
- Weakness, tiredness, and sleep problems
- memory problems, irritability, and feeling depressed
- Headache, le crams, and muscle aches
- Feeling nervous
- Dryness of the skin and the hair; hair loss
- Weight changes
Serious Side Effects of Synthroid
Synthroid may cause serious side effects. If the patient develops the following symptoms, the patient should call the physician immediately when the following symptoms occur:
- Decreased bone mineral and density which includes the symptoms of decreased height and the bones breaking easily.
- Heart problems
- And the severe allergic reaction
Adverse Reactions to Synthroid
Adverse reactions related to Synthroid therapy are primarily due to therapeutic overdosage, which may include the following:
- General: fatigue, increased appetite, weight loss, heat intolerance, fever, and excessive sweating
- Central nervous system: headache, hyperactivity, anxiety, irritability, emotional lability, insomnia
- Musculoskeletal: tremors, spasms and weakness of muscles
- Cardiovascular: palpitations, changes in heart rhythm, increases blood and heart pressure, myocardial infarction, and cardiac arrest.
- Respiratory: dyspnea
- Gastrointestinal: diarrhea, abdominal cramps, elevated liver function tests
- Dermatologic: hair loss, rash, and flushing
- Reproductive: impaired fertility, and irregular menstruation
Adverse Reactions of Synthroid in Pediatric Patients
Pseudo cerebri and slipped capital femoral epiphysis have been reported in children who have received Synthroid. Overdosage may cause craniosynostosis in infants and premature infants because of the closure of the epiphyses in children which will cause compromised adult height.
Hypersensitivity Reactions to Synthroid
Patients that are treated with thyroid hormone products may experience hypersensitivity reactions from the inactive ingredients of the medication. Hypersensitivity reaction to Synthroid may manifest as:
- Urticaria
- Pruritus
- Skin rash
- Flushing
- Angioedema
- Abdominal pain
- Nausea
- Vomiting
- Diarrhea
- Fever
- Arthralgia
- Serum sickness
- Wheezing
Cautions and Contraindications of Synthroid
Synthroid is contraindicated in patients with the following conditions:
- Acute myocardial infarction (MI) or cardiovascular events
- Hypersensitivity to levothyroxine
- Acute myocarditis
- Hyperthyroidism
- Thyrotoxicosis
- Cardiac arrhythmias
- Hypersensitivity to meat products (particularly beef)
- Adrenal insufficiency
Synthroid should be administered with caution due to the following:
- Nursing mothers (lactating)
- Ensure adequate dosage adjustment to prevent hypothyroidism from persisting or hyperthyroidism from developing. Initial doses are kept low to prevent heart rate, and blood pressure rises.
- Patients with compromised cardiac status who are receiving dose adjustments to meet oxygen demand
- Patients with cardiovascular disorders
- Patients receiving concomitant levothyroxine and sympathomimetic agent since it can cause coronary insufficiency
- Patients who are taking levothyroxine and a sympathomimetic medication concurrently
- Diabetic patients
- Patients presenting with hypopituitarism
- Patients with Addison’s disease
- Obese patients since it can cause life-threatening side effects
- Presence of untreated adrenal gland disorder
- Presence of thyrotoxicosis
- Patients with symptoms of heart attack
- Reported angina pectoris
- Presence of hypertension
- Patients with impaired kidney function
Drug Interactions with Synthroid
Synthroid can interact with other medications that affect the metabolism of the thyroid hormone, such as phosphate binders (e.g., calcium carbonate, ferrous sulfate), which binds to levothyroxine. Antacids may affect the uptake of Synthroid as gastric acidity is a proponent for decreased gut motility, often caused by hypothyroidism.
Catecholamines are also implicated as thyroid hormones increase their effect and responsiveness. Other drugs that could potentially alter its therapeutic response are the following:
- Cholesterol-lowering drugs such as cholestyramine. These decrease the levels of thyroid hormone (hypothyroidism) via inhibition of gastrointestinal absorption. The mechanism responsible is believed to be the formation of an insoluble complex with Synthroid,
- Calcium carbonate. Coadministration with Synthroid results in malabsorption of thyroid hormone due to drug binding.
- Antacids (containing magnesium or aluminum). These drugs decrease thyroxine (T4) absorption and can result in inadequate thyroid hormone replacement in patients receiving levothyroxine.
- Sucralfate. This reduces absorption of T4. These can cause hypochlorhydria decreasing Synthroid absorption.
- Sodium polystyrene sulfonate. This reduces absorption of Synthroid; therefore, sodium polystyrene sulfonate administration should be spaced out by at least four hours.
- Antibiotics (such as rifampicin which is used for the treatment of tuberculosis), These may accelerate the hepatic breakdown of levothyroxine, resulting in increased dosing needs. Modifies metabolism of T4.
- Bile Acid Sequestrants (e.g., cholestyramine). This reduces thyroid hormone levels, specifically T4, and inhibits levothyroxine absorption in the gastrointestinal tract.
- Proton Pump Inhibitors. Levothyroxine absorption is inhibited because PPI suppresses stomach acid secretion.
- Stomach acid reducers. Levothyroxine doses may need to be increased if stomach acid levels are low.
- Anti-inflammatory drugs. When combined with Synthroid, these drugs may cause a transient increase in FT4. After long-term use, patients are considered clinically erythroid, as evidenced by serum T4 and normal FT3 and thyroid-stimulating hormone concentrations. Thyroxine binding globulin and transthyretin binding of free thyroxine and triiodothyronine is inhibited by salicylates (e.g., diphenylhydantoin). Free T3 and T4 levels rise as a result of this.
- Antidiabetics. The increase in the insulin and oral hypoglycemia agent requirements (dosage adjustments) may be due to hormonal imbalance and past corticosteroid usage. Although, thyroid hormone levels also impact blood glucose presentation. Diabetic patients on Synthroid may compromise glucose control, and monitoring may be necessary to avoid deterioration.
- Sympathomimetics. This drug augments the effect of thyroid hormone or sympathomimetics since thyroid hormones may increase the risk of coronary insufficiency in patients taking sympathomimetic drugs.
- Tyrosine-kinase inhibitors. Some tyrosine kinase inhibitor or TKIs may cause hypothyroidism when taken with Synthroid.
- Anticonvulsants such as Carbamazepine. Stimulates liver microsomal drug-metabolizing enzyme (DME): cytochrome p-450 and NADPH cytochrome C reductase, influencing lipid concentrations. Alters metabolism of thyroid hormones T3 and T4.
- Cardiac glycosides such as digoxin. The therapeutic effect may become diminished as a result of conversion to the euthyroid state of patients with hypothyroidism.
- Ketamine. Adverse effects of ketamine and Synthroid (e.g., tachycardia, hypertension) may be exacerbated if they are taken at the same time. Maintain and closely observe these patients’ blood pressure and heart rate.
- Anticoagulants. Synthroid uptake increases when taken alongside anticoagulants. This is related to the increase in vitamin K-dependent factor catabolism caused by thyroid hormone, which increases anticoagulation. However, prolonged use can impede clotting factor synthesis. Warfarin enhances the degradation of vitamin K-dependent clotting factors and catabolism, which would increase the response of warfarin. Regular International Normalized Ratio (INR) monitoring should be performed. Increased hospitalizations and dosage modifications can result from long-term use.
- Antidepressants. Toxic effects and outcomes of Synthroid and other tricyclics, like amitriptyline, and tetracyclic depressants, may be enhanced when these medicines are administered together. The onset of action of tricyclic depressants is expedited, and intake of sertraline can increase the dosage threshold. Selective serotonin reuptake inhibitors can also induce a toxic or therapeutic effect. An increased incidence of cardiac arrhythmias and increased activity/stimulation of the central nervous system (CNS) are possible toxic implications.
- Drug-food interactions. Certain foods have anti-thyroid effects and impede their absorption, which may necessitate dose modifications. Foods such as nuts, soybean flour, walnuts, cruciferous vegetables, and cottonseed meal may bind with levothyroxine (Synthroid) and reduce its absorption from the gastrointestinal tract. Grapefruit has also been shown to delay its absorption due to the inhibition of certain enzyme activity. As a result, decreased drug concentrations can be seen in plasma.
- Drug-laboratory test interactions. When interpreting T4 and T3 levels, it is essential to take into account variations in TBG concentrations (owing to pathological conditions), as this can impact measurements. Increases in TBG concentration are seen in infectious hepatitis, pregnancy, and intermittent porphyria. Decreases in TBG are attributed to thyroxine-binding globulinemias.
- Steroids. These may reduce the conversion of T4 to T3:
- Estrogen-containing contraceptives, estrogens, and methadone. Increases TBG concentration.
- Androgens and glucocorticoids. Decrease TBG concentration.
- Cytokines. These are associated with transient hyper- and hypothyroidism. Transient, asymptomatic thyroiditis has been linked to interleukin-2.
Nursing Considerations for Patients on Synthroid
- Obtain a medical history and ask for any allergies, recent MI, presence of hypothyroidism, Addison’s disease, or thyrotoxicosis.
- Observe for any skin changes (e.g., presence of skin lesions, decrease in muscle tone)
- Evaluate thyroid function
- Perform blood pressure measurements using the apical pulse prior to medication
- Assess the presence of chest pain, weight loss, tremors, and insomnia, and report the findings as soon as possible.
- ● The daily dose progressively increases over the course of a few weeks.
- Administer on an empty stomach early in the morning or at least one hour prior to meals
- Monitor indications of insufficient or excessive dosage
- Educate patients on the necessity of long-term replacement for hypothyroidism.
- Synthroid is not indicated to suppress the benign thyroid nodules and nontoxic goiter in iodine-sufficient patients because there are no clinical benefits and overtreatment of Synthroid may induce hyperthyroidism.
- Synthroid is not indicated for the treatment of hypothyroidism during the subacute thyroiditis recovery phase.
Nursing Care Plan for Patients on Synthroid
Possible Nursing Diagnosis
- Acute Pain (Headache) related to the possible side effect of Synthroid
- Imbalanced Nutrition: Less Than Body Requirements related to the possible side effect of Synthroid
- Nausea related to the possible side effect of Synthroid
- Disturbed Sleep Pattern related to the long-term side effect of Synthroid
- Deficient Knowledge related to a new drug prescription
Nursing Assessment
| Synthroid Nursing Interventions | Rationale |
| Assess the patient for signs and symptoms of hypothyroidism. | To confirm the indication for administering levothyroxine (Synthroid). |
| Assess if the patient has any of the following conditions: Myocardial infarction (MI) Acute myocarditisHyperthyroidismThyrotoxicosisAcute myocarditisCardiac arrhythmiasAdrenal insufficiency | Synthroid is generally contraindicated in patients in patients with the said conditions as Synthroid may worsen them. |
| Check the patient’s allergy status. | Previous allergic reactions to levothyroxine (Synthroid) may render the patient unable to take it. Alternatives to Synthroid or levethyroxine in general should therefore be considered in case of allergy. |
| Assess if the patient is pregnant or lactating. | Pregnant and lactating/ breastfeeeding women can take Synthroid with caution. Synthroid is under pregnancy category A, as research shows that Synthroid is not teratogenic and is therefore not harmful for both the mother and baby. |
| Perform the necessary blood tests to determine thyroid function. | To confirm the indication for administering Synthroid. |
| Assess the patient’s mucous membranes and his/her ability to swallow. | To check for any potential problems with administration, hydration, and absorption. To ensure that the right form of the drug is given. |
| Assess the patient’s vital signs, particularly the blood pressure and heart rate. | Levothyroxine (Synthroid) may cause hypertension, tachycardia, and cardiac arrhythmias. |
Nursing Planning and Intervention
| Synthroid Nursing Interventions | Rationale |
| Administer Synthroid 30 to 60 minutes before meals, ideally at the same time each day whether day or night. | To ensure optimal absorption and therapeutic action of Synthroid, as well as to reduce possible side effects. Taking Synthroid with meals or on a full stomach may cause decreased absorption. |
| Conduct a medication review. Take caution that some medications such as aluminum-containing antacids, calcium carbonate, and sucralfate among many others, are not recommended to be taken with Synthroid, unless indicated by the physician. | Aluminum-containing antacids, calcium carbonate, and sucralfate and several other medications may interact with the action of Synthroid in the body. |
| Educate the patient about the action, indication, common side effects, and adverse reactions to note when taking Synthroid. Instruct the patient on how to self-administer Synthroid. | To inform the patient on the basic information on Synthroid, as well as to empower him/her to safely self-administer the medication. |
| Monitor the patient’s input and output, as well as episodes of nausea and vomiting. | Synthroid may cause nausea and vomiting, Early detection of these gastrointestinal side effects can start immediate interventions )e.g. antiemetics) as needed. |
| Discourage intake of grapefruit juice and soy-containing foods. | Grapefruit juice can decrease the level of absorption of Synthroid in the body. Soy-containing foods may also reduce the effectiveness of Synthroid. |
| Encourage reduction of alcohol consumption. | Alcohol causes direct toxicity on the cells of the thyroid gland, thereby affecting thyroid levels. This will affect the dosage of Synthroid that the patient requires. |
| Swallow the tablet whole. If unable to swallow, Synthroid can be crushed and dispersed in water. | Synthroid can be crushed for patients with difficulty of swallowing known as dysphagia. Crushing the tablet will not affect its effectiveness or potency. |
Nursing Evaluation
| Synthroid Nursing Interventions | Rationale |
| Ask the patient to repeat the information about Synthroid. | To evaluate the effectiveness of health teaching on Synthroid. |
| Monitor the patient’s blood pressure level and heart rate. | To ensure that Synthroid did not cause any hypertension or tachycardia. |
| Monitor the patient’s sleeping pattern. | To check if Synthroid has caused insomnia, which may happen especially to patients who have been taking the drug on a long-term basis. |
Nursing References
Ackley, B. J., Ladwig, G. B., Makic, M. B., Martinez-Kratz, M. R., & Zanotti, M. (2020). Nursing diagnoses handbook: An evidence-based guide to planning care. St. Louis, MO: Elsevier. Buy on Amazon
Gulanick, M., & Myers, J. L. (2022). Nursing care plans: Diagnoses, interventions, & outcomes. St. Louis, MO: Elsevier. Buy on Amazon
Ignatavicius, D. D., Workman, M. L., Rebar, C. R., & Heimgartner, N. M. (2020). Medical-surgical nursing: Concepts for interprofessional collaborative care. St. Louis, MO: Elsevier. Buy on Amazon
Silvestri, L. A. (2020). Saunders comprehensive review for the NCLEX-RN examination. St. Louis, MO: Elsevier. Buy on Amazon
