The objective of this study was to analyze the features of monotherapy with L-T4 in comparison with combined therapy with L-T4 and L-T3 in patients with primary hypothyroidism.

We conducted a randomized controlled study with a crossover design in 36 premenopausal women with overt primary hypothyroidism (reduced T4 concentration accompanied by increased TSH concentration at the time of initial diagnosis) who did not receive thyroid hormones. All patients were divided into two groups: patients from Group A (n=20) were randomized to L-T4 in dose of 1,6 μg per kg, followed by combined L-T4 and L-T3; and 16 women from Group B received the combination of L-T4+L-T3 (the dose of L-T4 was reduced by 25 μg and replaced with 12,5 μg of L-T3), followed by monotherapy with L-T4. The treatment periods lasted for 6 months. Patients were examined at baseline and at the end of each treatment period.Blood samples were obtained to measure serum TSH, free T4, free T3, lipid profile, osteocalcin. Urine samples were collected to measure urinary deoxypyridinoline / creatinine ratio. 24-hour ECG-monitoring, osteodensitometry and psychological testing were also performed at baseline and at the end of each treatment period.

Inclusion Criteria:

• Premenopausal women with overt primary hypothyroidism (reduced T4 concentration accompanied by increased TSH concentration at the time of initial diagnosis) who did not receive thyroid hormones

Exclusion Criteria:

• Peri- and postmenopause
• Pregnancy
• Major comorbidity
• Use of drugs that affect metabolism or bioavailability of thyroid hormones preparations.