The patient's symptoms of fatigue, weight gain, cold intolerance, dry skin, bradycardia, and delayed ankle reflexes are classic signs of hypothyroidism. The best initial lab tests are TSH and free T4; elevated TSH with low free T4 confirms primary hypothyroidism. TSH is the most sensitive marker to screen thyroid function because it rises in response to low thyroid hormone levels. Free T4 helps determine the thyroid hormone status. Diagnosing hypothyroidism early is important for timely treatment and symptom reversal.

Reference: Harrison's Principles of Internal Medicine, J. Larry Jameson et al., 20th Edition.