Hypothyroidism is defined by which change in thyroid hormones?

• A. Thyroid hormone deficiency with low T3 and T4
• B. Increased T3 and T4
• C. Normal T3 and T4 with symptoms
• D. High TSH only with normal T3/T4

Answer: (A)

Hypothyroidism is defined by insufficient production of thyroid hormones, which means low circulating levels of both T3 and T4. These hormones regulate the body’s metabolism, so when they are deficient, metabolic slowing occurs and symptoms can develop.

That’s why describing a deficiency with low T3 and T4 best captures the defining change. Increased T3 and T4 would indicate the opposite condition (hyperthyroidism). Normal T3 and T4 with symptoms wouldn’t meet the lab criteria for hypothyroidism. A high TSH with normal T3/T4 suggests subclinical hypothyroidism, where the hormone levels themselves aren’t reduced, so it doesn’t define hypothyroidism by hormone deficiency.