T3 Thyrotoxicosis

T3 thyrotoxicosis is thyrotoxicosis in which free triiodothyronine is raised, free thyroxine is normal and thyroid stimulating hormone is suppressed.

It accounts for a minority of hyperthyroid cases, and is more common in iodine deficient regions and in toxic nodular goitre. It can be an early or relapsing phase of Graves disease.

Causes are early or relapsing Graves disease, toxic adenoma, toxic multinodular goitre and rarely exogenous liothyronine or thyroid hormone excess. Relapse after antithyroid drugs often begins as T3 thyrotoxicosis.

Pathophysiology.

Hyperfunctioning tissue under chronic stimulation preferentially synthesises and secretes triiodothyronine, and peripheral conversion of thyroxine to triiodothyronine is increased. Free thyroxine stays within the reference range.

• Palpitations, tremor, weight loss, heat intolerance, sweating, anxiety and sleep disturbance.
• Goitre or nodules; eye signs if Graves disease.

Red flags.

• Atrial fibrillation, heart failure or chest pain.

• Suppressed thyroid stimulating hormone (under 0.1 mU/L in most) with raised free triiodothyronine and normal free thyroxine, on two occasions 4 to 6 weeks apart if mild.
• Repeat on a different assay platform and ask about biotin to exclude interference.
• Thyrotropin receptor antibodies. Thyroid scintigraphy or uptake scan if antibodies are negative, and ultrasound for nodules.
• Ask about exogenous liothyronine or weight loss preparations.

Differential diagnosis.

• Subclinical hyperthyroidism (free triiodothyronine normal).
• Non-thyroidal illness (free triiodothyronine is low, not high), drug effects (steroids, dopamine) and early thyroiditis.
• Factitious thyrotoxicosis from liothyronine.

• Treat by cause: Graves disease with carbimazole 15 to 40 mg daily, titrated to free triiodothyronine and thyroid stimulating hormone, then reduced; toxic adenoma or multinodular goitre with an antithyroid drug followed by radioiodine or surgery.
• Propranolol 10 to 40 mg three to four times daily for symptoms.
• Stop any exogenous liothyronine.
• Titrate to free triiodothyronine as well as free thyroxine, because thyroxine alone will appear normal.
• Block and replace with levothyroxine is an option once on a high dose antithyroid drug.

Special groups. Pregnancy: use propylthiouracil in the first trimester and carbimazole after, at the lowest effective dose.

Monitoring. Free triiodothyronine and thyroid stimulating hormone every 4 to 6 weeks during titration. Thyroid stimulating hormone may stay suppressed for months after control.

Referral and escalation. Endocrinology for all overt cases and for pregnancy.