From Dr. Lewis Blevins. Patient’s with pituitary and thyroid disorders may be required to make adjustments in thyroid hormone dosages since their glands cannot work together to make those adjustments in certain situations that affect thyroid hormone metabolism or the carrying capacity of thyroid hormone binding proteins. It is important to be aware that either starting or stopping a number of different medications might affect your thyroid hormone dose requirements. It’s always a good idea to ask your pharmacist if any new medications might interfere with your thyroid hormone replacement or other hormone medications that you’re taking. Educate yourself ……and teach your doctors!
Estrogens elevate thyroxine binding globulin (TBG) levels. Pregnancy and oral contraceptives as well as estrogen supplementation may elevate TBG levels. TBG is the main carrier protein for thyroid hormone in the blood stream. Elevations in thyroxine binding globulin mop up free T4. Normal people are able to compensate for this but the patient with hypopituitarism or hypothyroidism will usually require an increased dose of thyroxine. Conversely, after pregnancy, or following discontinuing estrogens one may be required to lower her dose of thyroid hormone replacement.
Androgens lower TBG levels. Some men treated with testosterone require a slight lowering of their thyroid hormone dosage while those who discontinue testosterone may need to increase the dose.
Some patients taking growth hormone require minor alterations in their thyroid hormone dosage.
Somatostatin analogs can unmask mild central hypothyroidism and create a need to take thyroid hormone or else increase the dose requirement.
Steroid medications, especially in the doses that are above replacement doses, inhibiting conversion of T4 to T3. A rare patient taking thyroid hormone would require T3 supplementation.
Narcotics increase TBG levels. Patient who take narcotics often require marked increases in thyroid hormone dosages whereas a lowering is necessary if narcotics are discontinued.
Al3+ containing drugs (Antacids, Sucralfate) can bind thyroid hormone and prevent its absorption leading to an increased dose requirement.
Iron and calcium can also bind thyroid hormone in the intestinal tract and prevent its absorption. Dose requirement increases are usually not necessary for patient’s who take thyroid hormone separate from iron and calcium pills.
Cholestyramine, a drug used to treat hypercholesterolemia, can also bind thyroid hormone and prevent its absorption.
Amiodarone causes complex effects on thyroid hormone secretion and metabolism. Some patients develop hypothyroidism and others develop hyperthyroidism. Amiodarone can block the conversion of T4 to T3 necessitating supplemental T3 therapy in hypothyroid patients on thyroxine.
Rifampin and Dilantin both have complex effects on the pituitary thyroid axis. A rare patient taking one of these medications requires an increase in dose of thyroid hormone.
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