3. Pregnancy: may levothyroxine requirement by
40% or more beginning in the 1st trimester,
HYPOTHYROIDISM independent of Fe administration.
CLINICAL MANIFESTATIONS 4. Misc Drugs: Zoloft, estrogens, calcium, and
Fatigue and lethargy lovastatin, whose effects on levothyroxine
Dry skin requirements are uncertain.
Slight weight gain Increased requirements occur in ♀ taking androgens
Cold intolerance (e.g., danazole [Danocrine]).
Heavy menstrual periods Levothyroxine should be taken on an empty stomach
w/out any food or other medications.
Although different brands are thought to be identical,
brand interchange is often associated with
significant TSH alterations. When a new brand is
selected, thyroid function must be rechecked.
Bradycardia Other Preparations:
Diastolic HTN Liothyronine: rapid onset, high peak, and low trough
Cold skin concentrations, as well as the difficulty in thyroid
Nonpitting edema (myxedema) function test interpretation associated with its use, is
Hair loss usually not recommended for chronic therapy.
Delay in DTR relaxation phase. Rapid onset of action may be advantageous in
Lab Evaluation hypothyroid emergencies.
Desiccated thyroid: 1 grain of USP Thyroid contains ~
TSH and thyroxine, T4 concentration have an inverse
40 mug of levothyroxine and 10 mug of T3 . Doses
logarithmic relationship. A 50% in free T4 causes prescribed range from one-half to three grains.
a 90-fold in TSH.
Normal TSH: no additional studies. Subclinical Hypothyroidism
TSH: free T4 concentration or index is performed. TSH and a normal free T4 SCH affects 10 to 20% of
Measurement of T-3 concentration is rarely necessary. those older than 65 years of age.
Causes: Other causes of TSH
untreated adrenal insufficiency
Subacute Thyroiditis: recovery from severe illness.
Iodine deficiency: Not a problem in the United States
but is the most common worldwide cause of Progression to Hypothyroid: Young pts w/SCH are at
goitrous hypothyroidism. risk hypothyroid over time. Some pts
Drugs: spontaneously return to Nl
Inhibit thyroid hormone biosynthesis: PTU or High prevalence of antithyroid Ab HT.
methimazole TSH and (+) Ab become frankly hypothyroid at the
rate of 4.3%/yr.
Thyroid hormone release: lithium
Both: iodides, and Amiodarone. Either TSH or antithyroid Ab alone hypothyroid
Atrophic Hypothyroidism: Hypothyroidism w/out incidence of 2%/yr.
goiter iatrogenic due to radioiodine therapy of
Graves' hyperthyroidism, surgery, or neck external CV Risk: TSH > 10 LDL elevation. Treatment will
radiation for malignancy. LDL by an average of 14, corresponding to a 28%
Autoimmune thyroid atrophy: profound in CV risk.
hypothyroidism, in some cases due to TSH Therapy is recommended for SCH:
receptor-inhibiting Ab. TSH > 10,
symptoms suggestive of hypothyroidism
Levothyroxine Therapy strongly positive thyroid antibodies
Goal symptomatic improvement and TSH absence of cardiac disease
normalization. younger age.
[TSH] slightly or normal range (0.5 to 5 muU per Follow pts w/TSH 5 and 10 who are asymptomatic,
mL) may be followed in asymptomatic patients. older, antibody negative, and with concomitant
[TSH] <0.1 or <0.01 muU per mL) may thinning heart disease.
bones or A fib and is best avoided for most
Levothyroxine: (Levothroid, Levoxyl, Synthroid)
QD dosing. 7-day half-life. Occurs when a profoundly hypothyroid patient becomes
~ 70% of an oral dose is absorbed. The full replacement severely ill.
dose of is 1.6 mug per kg per day. Meaningful TSH Clinical Presentation:
is measured after 6 or more weeks at a given Symptoms of hypothyroidism
dosage. Altered mental status: lethargy, psychosis, confusion,
Initial therapy: coma
younger pts: near full replacement
Precipitating event: UTI, PNA, flu, cold exposure,
older patients or those with or at risk for heart dz:
narcotics, sedatives, hospitalization, surgery,
doses (e.g., 12.5 to 25 mug per day). trauma, CVA, hypoglycemia, CO2 narcosis, drug
Altered Thyroid Hormone Requirements overdose, diuretics
When compliance is variable, serum TSH may remain
elevated despite escalating levothyroxine dosages. Approximately half of these patients become comatose
Increased requirements occur with after hospital admission, possibly because of
1. levothyroxine metabolism: rifampin, phenytoin, excessive fluid administration, sedating drugs, or
carbamazepine, and phenobarbital. Requirements unrecognized sepsis.
may 2x. Treatment:
2. Interfere with absorption: iron (including Levothyroxine IV until able to take PO meds
multivitamins containing iron), Maalox, sucralfate, Liothyronine IV: Severely ill patients are unable to
and cholestyramine. convert T4 to T3
Stress doses of steroids: steroid deficiency may
accompany profound hypothyroidism.
Although hypothyroidism secondary to pituitary or
hypothalamic disease is uncommon, it deserves
special consideration. Adrenocorticotropic hormone
(ACTH) and other pituitary hormone deficiencies
may accompany TSH deficiency.
Disparity between a very low free T4 with minimal TSH
elevation must alert the clinician to the dangers of
Treatment: cortisol should be administered with
levothyroxine. Unfortunately, no early warning
system exists to detect mild central hypothyroidism.
With known pituitary or hypothalamic disease, a low
normal free T4 suggests hypothyroidism.
TSH is not useful to monitor therapy of central
hypothyroidism; clinical features and serum free T4
must be followed.
Euthyroid Sick Syndrome
T4: falls last
TSH: Nl to
Pts are not producing enough T3. This is due to excess
rT3. This is produces a feedback mechanism to
production of all other thyroid hormones.
Found mainly in pts w/poor liver function. The liver
metabolizes rT3. Liver failure metabolism of
rT3 rT3 inhibition of production of