It has been shown that insulin might be involved in the pathogenesis of thyroid growth.

Objective To evaluate the impact of IR and metformin use on the volume of benign thyroid nodules (TNs).

Methods A randomized clinical trial to placebo (P) or MTF use. Previous fine needle aspiration confirming the diagnosis is necessary to inclusion. Patients will receive similar tablets of MTF and placebo and instruct to take 3 tablets/day of MTF (500mg/tablet). Thyroid volume, as TN volume, will be assess by ultrasound, both in the beginning, six months and one year after randomization, by the same researcher blinded regarding location group. Blood samples to measure: TSH, FT4, TPO-Ab, lipid profile, glucose and insulin were done after 8h fasting.

It has been shown that insulin might be involved in the pathogenesis of thyroid growth. Previous studies suggest that metformin (MTF) therapy decreased thyroid volume and nodule size in subjects with insulin resistance (IR).

Objectives To evaluate the impact of IR and metformin use on the volume of benign thyroid nodules (TNs).

Methods A randomized clinical trial to placebo (P) or MTF use. Previous fine needle aspiration confirming the diagnosis is necessary to inclusion. Exclusion criteria were: nodules with a predominantly cystic pattern, pregnancy, diabetes, acromegaly, previous use of MTF, levothyroxine, corticoid or any weight loss medication in the past six months, hepatic or cardiac insufficiency, creatinine levels > 1.4 mg/dL and MTF intolerance doses >1.0 gram/day. Patients will receive similar tablets of MTF and placebo and will be instructed to take 3 tablets/day of MTF (500mg/tablet). Thyroid volume, as TN volume, will be accessed by ultrasound, both in the beginning, six months and one year after randomization, by the same researcher blinded regarding location group. Blood samples to measure: TSH, FT4, TPO-Ab, lipid profile, glucose and insulin were done after 8h fasting.

Inclusion Criteria:

• patients with benign nodules diagnosed by fine needle aspiration biopsy (FNAB)

Exclusion Criteria:

• Nodules with a predominantly cystic pattern
• Eggshell calcification
• Coalescent thyroid nodules (not suitable for size analysis)
• Pregnancy
• Diabetes
• Acromegaly
• Previous use of MTF, levothyroxine, corticoid or any weight loss medication in the past six months
• Hepatic or cardiac insufficiency
• Creatinine levels > 1.4 mg/dL
• MTF intolerance doses < 1.0 gram/day

The same researcher blinded regarding location group will access thyroid volume and thyroid nodule volume by ultrasound, in the beginning, six months and one year after randomization