T4 is converted to T3 (4x more potent), T4 inhibits secretion of TSH, a negative feedback loop is formed
Labs: T4: 0.8 - 1.7 mcg/dL, TSH: 0.3 - 3 uIU/mL, TSH is the most reliable test of thyroid function, it is monitored every 4-6 weeks until normal level, then 4-6 months, then annually.
When thyroid hormones decrease, body metabolism slows down.
Levothyroxine is safe in pregnant women, and may require higher dose.
Signs symptoms: weight gain, dry skin, brittle nail, depression, constipation, bradycardia, myxedema (emergency of poor circulation), goiter (can be presented in hyperthyroidism).
Myxedema coma is a life-threatening condition characterized by poor circulation, hypometabolism, it occurs when hypothyroidism is left untreated for long time. The initial treatment is IV levothyroxine.
First line: Levothyroxine (T4), (Synthroid, Levoxyl, Unithroid), half-life 6-7 days.
Warnings: CV diseases (↓ dose), ↓ bone mineral density (osteoporosis).
Does it have to be taken on an empty stomach? Not really. It’s true that absorption is higher on an empty stomach or at bedtime, and away from interference with calcium, iron, multivitamins with minerals, etc. But maintaining consistent is really the key. Emphasize taking the same way with regards to meals, meds, and time of the day.
PO : IV = 2: 1 (not lipophilic), dilution with normal saline, shake well use immediately, room temp.
Second line (Desiccated thyroid extract, Liothyronine, Liotrix): less predictable, shorter half-life leading to fluctuation in T3 level.
Desiccated thyroid extract (Armour Thyroid, NP Thyroid): also called “natural thyroid”, contains variable amount (therefore not recommended) of T3&4, combo might ↑ CV risk.
Liothyronine (Cytomel, Triostat): highly protein bound, contains T3, as a adjunct to levothyroxine in some cases.
↓ absorption due to binding: Cholestyramine, cations (Ca, Mg, Al etc.), sucralfate, sevelamer, sodium polystyrene sulfate (SPS, Kayexalate), orlistat.
↓ conversion of T4 to T3: beta blockers, PTU, amiodarone.
Thyroid hormone is highly protein bound, avoid drugs that can displace protein binding: phenytoin, heparin, salicylates, NSAIDs.
Hyperthyroidism:
Elevation of T4, low level TSH.
S/Sx: accelerated metabolism, weight loss, irregular heartbeat, irritability, sweating, tachycardia, osteoporosis, goiter (present in both hypo and hyper), exophalmus (bulging eyeball), Afib, fracture.
Causes: Graves’ disease (autoimmune disorder stimulates thyroid gland to produce too much T4), drugs (iodine, amiodarone, interferon, radiocontrast media)
Treatment: destroy glad via radioactive iodine or surgery.
Beta blocker: control symptoms (reduce palpitation, tachycardia).
PTU or methimazole is used temporarily before surgery, it takes 1-3 months of high dose antithyroid treatment to control symptoms, then reduced to prevent hyperthyroidism.
PTU (propylthiouracil):
MOA: inhibits synthesis of thyroid hormone, and peripheral conversion of T4 -> T3.
Warnings: liver damage (hepatitis), preferred in 1st trimester (methimazole for remaining trimesters); agranulocytosis (monitor WBC), DILE (lupus).
First line in thyroid storm (occurs in stressful events: infection, MI, DM, labor, lack of treatment) + SSKI (saturated solution of potassium iodine)
50 – 150 mg Q8H until euthyroid.
Methimazole (Tapazole)
Warnings: liver damage gives with food, infections, preg cat D, preferred in 2nd and 3rd trimesters.
5 mg Q8H until euthyroid, then 5-15 mg daily.
Iodine treatments
MOA: temporarily inhibit secretion of thyroid hormone.
