What is the typical lab pattern in primary hypothyroidism and the standard initial therapy?

• A. Low TSH with high free T4; methimazole.
• B. Normal TSH with normal T4; no treatment.
• C. Elevated TSH with low free T4; treat with levothyroxine (synthetic T4) dosage adjusted to TSH.
• D. Elevated TSH with high free T4; propylthiouracil.

Answer: (C)

When primary hypothyroidism is present, the thyroid fails to produce enough hormone, so the pituitary signals for more with an elevated TSH, while free T4 remains low. The standard initial treatment is levothyroxine, a synthetic T4. Start with a dose around 1.6 mcg/kg per day in healthy adults and adjust based on the TSH target, rechecking the TSH about 6–8 weeks after starting or changing the dose. In older adults or those with heart disease, begin lower (about 12.5–25 mcg per day) and titrate up carefully. If the pattern showed low TSH with high T4, that would point to hyperthyroidism, not hypothyroidism, and the antithyroid drugs used there (like methimazole or propylthiouracil) would not be appropriate here. Similarly, a normal TSH with normal T4 suggests no treatment, and an elevated TSH with high T4 does not fit primary hypothyroidism.