A patient has clinical hyperthyroidism with raised T3 and T4, yet the TSH is also ELEVATED. What is the most likely cause?

Correct answer: C. A TSH-secreting pituitary adenoma

• A. HASHIMOTO THYROIDITIS causes primary HYPOthyroidism — low T3/T4 with HIGH compensatory TSH. The TSH fits, but the hormone levels and the clinical picture do not.
• B. GRAVES DISEASE is PRIMARY hyperthyroidism: TSI autoantibodies stimulate the TSH receptor directly, so T3/T4 rise and TSH is SUPPRESSED to near-undetectable levels. That is the opposite pattern.
• C. CORRECT. Normally high thyroid hormone suppresses TSH by negative feedback. When BOTH the thyroid hormones and TSH are elevated, the feedback loop has been overridden by an autonomous source of TSH — a TSH-SECRETING PITUITARY ADENOMA (thyrotroph adenoma), causing SECONDARY (central) hyperthyroidism. The pituitary is driving the thyroid rather than responding to it.
• D. IODINE DEFICIENCY causes hypothyroidism and goitre with raised TSH and low thyroid hormones — again the wrong hormone direction.

Note: Read TSH and free T4 together as a pair. Low TSH + high T4 = primary hyperthyroidism. High TSH + low T4 = primary hypothyroidism. High TSH + high T4 = secondary (pituitary) hyperthyroidism. Low TSH + low T4 = secondary (pituitary) hypothyroidism.