Drug Interaction:
Thyroid Hormones include- natural and synthetic derivatives. The natural products , desiccated thryroid and thyroglobulin are derived from beef or pork. Synthetic derivatives include Levothyroxine , Liothyroxine and Liotrix.
Refer Thyroglobulin
Interacting drugs- summary
+ Thyroid hormones
Cholestyramine and Colestipol
loss of efficacy of thyroid hormone and potential hypothroidism. Administer
4 to 6 hrs apart
Estrogen
estrogen increase TBg and may therefore decrease with response to
thyroid hormone therapy in patients with a nonfunctioning thyroid gland.
Anticoagulants
the anticoagulant action is increased, a decreased dose may be necessary
Betablockers
action of particular betablocker may be impaired when the hypothyroid
patient is converted to the euthyroid state
Digitalis glycosides
serum digitalis levels are reduced in hyperthyroidism or when the
hypothyroid patient is converted to the euthyroid state. Therapeutic
effects of digitalis glycosides may be reduced
Theophyllines
decreased theophylline clearance can be expected in hypothyroid
patients, clearance returns to normal when euthyroid state is acheived
Indication:
Thyroid Hormones include- natural and synthetic derivatives. The natural products , desiccated thryroid and thyroglobulin are derived from beef or pork. Synthetic derivatives include Levothyroxine , Liothyroxine and Liotrix.
Refer Thyroglobulin
Adverse Reaction:
Cardiovascular- palpitations, tachycardia, arrhythmias, angina pectoris, cardiac arrest.
CNS- tremors, headache, nervousness, insomnia,
GI- diarrhea, vomiting, .Gastric intolerance may occur rarely in patients highly sensitive to beef or pork products or corn.
Hypersenti vity- allergic skin reactions (rare)
Miscellaneous- weight loss, menstrual irregularities, sweating, heat tolerance, fever
Contra-Indications:
Cardiovasular diseases unless thyroid replacement therapy is clearly indicated. Uncorrected adrenal insufficiency.
Special precautions:
Overdosage or too rapid increase in dosage may result in signs and symptoms of
hyperthyroidism.
Monitoring-
Treatment of patients with thyroid hormines requires the periodic assessment of throid status by means of appropiate laboratory tests.
Decreased bone density- long term levothyroxine therapuy has been asspciated with decreased bone density in the hip and spine in pre- and postmenopausal women. It may be beneficial to obtsain a basal bone density measurement then monitor closely for osteoporosis development.
Tatrazine sensitivity- some ofthese products tatrazine which nay cause allergic-type reactions (including bronchial asthma) in suseceptible individuals.
Warnings-
Obesity has been treated with thyroid hormones.In euthyroid patients hormonal replacement doses are ineffective for weight reduction. Larger doses may produce serious or even life-threatening toxicity, particularly when given with sympathomimetic such as anorexiants.
Cardiovascular disease- use caution when integrity of the cardiovascular system, paricularly the coronary arteries, is suspect. The development of chest pain or other worsening of cardiovascular disease requires a decrease in dosage.
Endocrine disorders- thyroid hormone therapy in patients with concomittant diabetes mellitus or insipidus or adrenal insufficiency (Addisons disease) exacerbates the intensity of their symptoms. Appropiate adjustments in the therapy of these concomittant endocrine diseases are required.
Morphologic hypogonadism and nephrosis- rule out prior to initiating therapy
Myxedema- patients with myxedema are particularly sensitive to thyroid preparations. Begin treatment with small doses and gradual increments.
Hyperthyroid effects- in rare instances the administration of thyroid hormone may precipitate a hyperthyrpoid state or may aggravate existing hyperthyroidism
Pregnancy- do not discontinue thyroid replacement therapy in hypothyroid women during pregnancy.
Lactation- excercise caution when thyroid is admoinistered to a nursing woman.
Children- in infants excessive doses of thyroid hormone preparations may produce craniosynostosis.
Dosages/ Overdosage Etc:
Indications:
Hypothyroidism
Dosage:
Institute therapy with low doses. Usual staring dose is 30 mg with increments of 15mg every 2 to 3 weeks.
Maintenance dose- 60 to 120 mg/day.
Overdosage-
These agents rarely result in clinical toxicity
Symptoms
These agents rarely result in clinical toxicity
Chronic excessive dosage may produce signs and symptoms of hyperthyropidism eg. headache, irritability, nervousness, sweating, tachycardia, increased bowel motility, menstrual irrgularities, palpitatins, vomiting, psychosis, seizures, fever.
Angina and CHF may be induced or aggravated.
Shock may develop. complications may include cardiac failure, and arrhthmias, which could be fatal.
Massive overdosage may result in symptoms resembling thyroid storm.
Treatment
1. Reduce dosage or temporarily discontinue therapy.
2. Reinstitute treatment at a lower dosage.
3. In healthy individuals, normal hypothalmic- pituitary thyroid axis function is restored in 6 to 8 weeks after thyroid suppression.
4. Therapeutic regimens are not justified in asymptomatic patients.
Missed dose-
1. If you miss a dose of this medicine, take it as soon as possible.
2. However, if it is time for your next dose, skip the missed dose and go back to your regular dosing schedule.
3. Do not double doses.
4. If you miss 2 or more doses in a row or if you have any questions about this, check with your doctor.
Other Information:
Disorders of thyroid function tests- ( 386 )
Drugs causing adverse reaction- thyroid function tests
1. Oral contraceptives
2. Bromosulphalein
3. Phenindione
4. Iodides
5. Tolbutamide
6. Chlorpropamide
7. Lithium
8. Acetazolamide
9. Gold salts
10. Dimecaprol
11. Clofibrate
12. Phenothiazines ( long term )
13. Phenylbutazone
14. Sulfonamides
15. Phenytoin
Patient Information:
1. Replacement therapy is to be taken for life, except in cases of transcient hypothyroidism, usually associated with
thyroditis, and in those receiving a trial of the drug.
2. Take a single daily dose preferably before breakfast
3. Brand interchange- Do not change from one brand of this drug to another without consulting your pharmacist or
physician. Products manufactured by different companies may not be equally effective.
4. Do not discontinue medication except on advice of a physician
5. Notify physician if headache, nervousness, diarrhea, excessive sweating heat intolerance, chest pain, increased
pulse rate, palpitations(symptoms of hyperthyroidism) or any usual event occurs
6. Partial loss of hair may be experienced by children in the first few months of therapy, but this usually a
transcient phenomenon that result in later recovery
7. Not for use as primary or adjunctive therapy in a weight control program
8. If levothyroxine is taken on an empty stomach, absorption is increased.
Pharmacology/ Pharmacokinetics:
Pharmacology-
The mechanisms by which thyroid hormones exert their physiologic action are not clearly unserstood. It is believed that most of their efects are exerted through control of DNA transcription and protein synthesis.
Pregnancy and lactation:
Pregnancy- Do not discontinue thyroid replacement therapy in hypothyroid women during pregnancy.
Lactation- Excercise caution when thyroid is admoinistered to a nursing woman
. Children- In infants excessive doses of thyroid hormone preparations may produce craniosynostosis.
