This study will examine how two thyroid preparations-levothyroxine (T4) and liothyronine
(T3)-affect fat and cholesterol metabolism, blood sugar regulation, and thyrotropin
secretion in patients who have had their thyroid gland removed. Results of the study may
help in the development of better therapies to optimize blood sugar and cholesterol levels
in some patients.
Patients 18 years of age or older who have had most or all of their thyroid gland removed
and are taking long-term thyroid hormone medication may be eligible for this study after
Thyroid hormone action plays an important role in the regulation of many physiologic
processes, among them glucose and lipid metabolism. Interestingly, the clinical presentation
of thyroid dysfunction is extremely variable, with relatively poor correlation between
circulating hormone levels and clinical features. This finding suggests that the local,
intracellular concentration of the active hormone liothyronine (T3), regulated by peripheral
conversion of the pro-hormone levothyroxine (T4), is an important determinant in the
maintenance of the thyroidal homeostasis.
The aim of the present study is the evaluation of the role of peripheral thyroid hormone
conversion in the regulation of glucose and lipid metabolism by assessing the differential
response to T4 or T3 treatment in subjects devoid of endogenous thyroid hormone production.
T3 administration bypasses peripheral metabolism and therefore will allow us to assess the
role of the peripheral thyroid hormone conversion in the regulation of the hormone action at
the end-organ level.
Fifty hypothyroid subjects will be initially randomized to either of the thyroid hormone
replacements liothyronine (T3) or levothyroxine (T4) treatment period (one arm cross-over
design, with treatment sequence randomized), aimed to maintain serum TSH levels greater than
or equal to 0.5 less than or equal to 1.5 mU/L, indicating full replacement. After a 30-day
period of steady-state replacement the study subjects will be admitted to the Clinical
Center and, after a three-day period of stabilization and an overnight fast, will undergo
the following tests: escalating dose TRH stimulation test, indirect calorimetry, graded
exercise tolerance test, DEXA scan, and echocardiogram.
Patients will also undergo skeletal muscle biopsy and subcutaneous adipose tissue biopsy and
microdialysis, as well as a two-step euglycemic hyperinsulinemic clamp with measurement of
splanchnic gluconeogenesis. Fasting venous blood samples will be collected for the
determination of the parameters of lipid, glucose and energy metabolism.
After discharge, the patients will switch to the other form of thyroid hormone replacement
therapy (second period) . The therapy will be adjusted in order to achieve the same
therapeutic goal for TSH concentrations (greater than or equal to 0.5 less than or equal to
1.5 mU/L), analogous to that achieved during the first phase of the study (TSH less than or
equal to 0.5 mU/L difference between T3 and T4 phases). After reaching a 30-day period of
steady-state replacement, study subjects will be re-admitted to the Clinical Center and the
previously described evaluation procedures will be repeated.
- INCLUSION CRITERIA:
Age greater than or equal to 18 years, male or female.
History of total or near total thyroidectomy or hypothyroidism on replacement therapy.
For non-thyroidectomized patients, at least three-year history of replacement therapy (at
least 1.2 mcg/Kg LT4/body weight), and less than 5% uptake at 24H on (123)I thyroid scan
while on replacement therapy.
Written informed consent.
BMI less than or equal to 20 or greater than or equal to 30 kg/m(2).
Metastatic thyroid cancer or history of thyroid cancer with high risk of recurrence
requiring suppressive thyroid hormone therapy (Singer 1996).
Significant thyroid residual greater than 1 mL as measured by ultrasound (limited to
thyroidectomized patients) or greater than 5 percent uptake at 24H on (123)I thyroid scan
while on replacement therapy (limited to hypothyroid patients not undergone total
History or symptoms compatible with cardiovascular disease, including paroxysmal
supraventricular tachycardia, atrial fibrillation, syncopal episodes or use of
prescription medications for heart conditions, including antihypertensives.
Allergy to lidocaine, isoproterenol, TRH, levothyroxine, liothyronine, Tylenol #3,
Pregnancy or unwillingness to use non-hormonal contraception during the study.
Use of hormonal contraceptives or estrogen replacement therapy.
Use of tobacco (smoking, chewing) for the two weeks preceding the hospital admissions
Diabetes mellitus, either type I or II.
Hypercholesterolemia (serum levels greater than or equal to 240 mg/dL),
hypertriglyceridemia (plasma levels greater than or equal to 220 mg/dL) and/or use of
Liver disease or ALT serum level greater than two fold the upper laboratory reference
Renal insufficiency or estimated creatinine clearance less than or equal to 50 mL/min.
Use of medications/supplements/alternative therapies known to alter thyroid function.
Current history or symptoms compatible with psychosis including major depression
(including history of hospitalization for depression, history of attempted suicide,
history of suicidal ideation).
Use of antipsychotic medications
History of drug or alcohol abuse within the last 5 years; current use of drugs or alcohol
(CAGE greater than 3).
Keloid formation (relative to skeletal muscle and subcutaneous adipose tissue biopsies).
Current or previous clinically significant (requiring medical/surgical intervention)
extrathyroidal manifestations of autoimmune thyroid disease (dermopathy, ophthalmopathy,