Thyrotoxicosis Differential, Thyroiditis & Graves Orbitopathy

Confirming thyrotoxicosis and finding the source

Two terms. Thyrotoxicosis is excess circulating thyroid hormone from any source; hyperthyroidism is the subset in which the gland itself overproduces. Only overproduction responds to synthesis blockers or radioiodine, so naming the mechanism is the whole game.

Biochemistry. A suppressed TSH with a high free T4 (usually with a high T3) confirms primary thyrotoxicosis. A high T3 with a normal free T4 (T3-toxicosis) is typical of early Graves disease and autonomous nodules. While the cause is sorted out, a β-blocker (propranolol, atenolol) controls tremor, palpitations, and tachycardia; diltiazem or verapamil substitutes when β-blockers are contraindicated.

Etiology. A thyrotoxic patient with a diffuse goiter and obvious orbitopathy needs no further proof of Graves disease. Otherwise, measure TSH-receptor antibodies (TRAb; the stimulating fraction is TSI): a positive result establishes Graves. If TRAb is negative or the picture is atypical (nodular gland, painful neck, recent iodine exposure), obtain a 24-hour radioactive iodine uptake (RAIU) and scan; radioiodine studies are avoided in pregnancy and lactation. Alternatives are Doppler ultrasound (high flow in Graves, low in destructive thyroiditis) and the total T3 (ng/dL) to T4 (μg/dL) ratio, where a value above 20 favors overproduction because an overactive gland releases proportionally more T3.

Graves disease. The most common cause of hyperthyroidism in iodine-sufficient regions, typically in women aged 20 to 50, with a diffusely enlarged, hypervascular gland.

Lid lag and a staring gaze reflect adrenergic overactivity and occur with any cause of thyrotoxicosis. Proptosis, pretibial myxedema, and thyroid acropachy are antibody-mediated and point specifically to Graves disease.

Thyroiditis and other low-uptake causes

Why uptake is low. In destructive thyroiditis, inflamed follicles spill stored T4, T3, and thyroglobulin into the blood. TSH falls, the damaged gland makes no new hormone, and it traps almost no radioiodine.

Subacute granulomatous (de Quervain) thyroiditis. Follows a viral upper respiratory infection: fever, malaise, and a firm, exquisitely tender goiter with pain radiating to the jaw or ear. ESR (often >50 mm/h) and CRP are high and RAIU is near zero; histology shows granulomas with multinucleated giant cells. Treat pain with NSAIDs; use prednisone (about 40 mg/day, tapered over several weeks) for severe pain or NSAID failure.

Painless (silent) and postpartum thyroiditis. Autoimmune lymphocytic variants related to Hashimoto thyroiditis. The postpartum form begins within 12 months of delivery or miscarriage, is more likely with TPO antibodies or type 1 diabetes, tends to recur with later pregnancies, and leaves a lasting risk of permanent hypothyroidism, so check TSH yearly. Positive TRAb and high Doppler flow instead suggest postpartum Graves; radioiodine testing is skipped in a nursing mother.

Shared management. The thyrotoxic phase is leakage of preformed hormone, so thionamides (which block new synthesis) and radioiodine (which the gland cannot take up) are useless. Give a β-blocker only and recheck thyroid tests every 4–8 weeks to catch the next phase.

Exogenous thyroid hormone. Factitious ingestion (weight loss, health-care workers) or excess prescribed levothyroxine gives low RAIU, no goiter, and a low serum thyroglobulin, because the gland is quiescent; thyroiditis raises thyroglobulin. Check anti-thyroglobulin antibodies too, since they interfere with the assay.

Iodine load (Jod-Basedow). Iodinated CT contrast, amiodarone, or kelp supplements give autonomous nodules the substrate to overproduce, especially in a nodular goiter from an iodine-deficient region. Uptake is low because the tracer is diluted in a large iodide pool; 24-hour urine iodine is high.

Struma ovarii. An ovarian teratoma composed mostly of thyroid tissue: low neck uptake, pelvic uptake on a whole-body scan, and cure by resection.

hCG-mediated thyrotoxicosis. hCG shares TSH's α-subunit and has a similar β-subunit, so it weakly stimulates the TSH receptor. Gestational transient thyrotoxicosis appears near the hCG peak at 10–12 weeks, often with hyperemesis gravidarum, without goiter or eye disease and with negative TRAb. It fades as hCG falls in the second trimester, so care is supportive (fluids, antiemetics, a brief β-blocker) without antithyroid drugs. Molar pregnancy and choriocarcinoma produce far more hCG and frank hyperthyroidism that resolves once the tumor is evacuated or treated.

Treating Graves disease

Thionamides. Every symptomatic patient also receives a β-blocker. Methimazole and propylthiouracil (PTU) inhibit thyroid peroxidase, blocking organification and coupling. Methimazole is preferred (once daily, less severe hepatotoxicity). A course runs about 12–18 months and is stopped if TRAb has normalized; relapse is common, especially with a large goiter, high TRAb, or continued smoking.

PTU niches. Methimazole is teratogenic in early pregnancy (aplasia cutis, choanal and esophageal atresia), so PTU is used in the first trimester and methimazole resumes afterward to limit PTU liver risk. PTU is also favored in thyroid storm because it additionally blocks peripheral T4→T3 conversion. In pregnancy, use the lowest dose that keeps free T4 at or just above the upper limit of normal, because thionamides cross the placenta and can cause fetal hypothyroidism.

Agranulocytosis (ANC <500/μL) usually strikes within the first 3 months of either thionamide. Fever or sore throat means stop the drug and obtain a CBC with differential; never switch to the other thionamide, because cross-reactivity is common. Get a baseline CBC and liver tests before starting.

Radioactive iodine. Oral I-131 is concentrated by the overactive gland and destroys it over the following months. Confirm a negative pregnancy test and advise avoiding conception for about 6 months. Because RAI can trigger or worsen orbitopathy, particularly in smokers, give prophylactic oral prednisone with mild active eye disease and avoid RAI in active moderate–severe or sight-threatening disease. Hypothyroidism is the intended endpoint: check thyroid tests every 4–6 weeks and start levothyroxine when it appears.

Thyroidectomy. Near-total or total thyroidectomy suits the indications in the table; in pregnancy it is done in the second trimester. Prepare with methimazole to euthyroidism, a β-blocker, and potassium iodide (SSKI or Lugol solution) for about 10 days to shrink gland vascularity. Afterward, watch for hypocalcemia from parathyroid injury and hoarseness from recurrent laryngeal nerve injury, and start levothyroxine.

Graves orbitopathy

Pathogenesis. TRAb bind TSH receptors on orbital fibroblasts, which also signal through the IGF-1 receptor. Activated fibroblasts secrete hydrophilic glycosaminoglycans and differentiate into fat. Extraocular muscles enlarge in a fusiform pattern with tendon sparing on CT or MRI, the inferior rectus most often, then the medial rectus, causing restricted gaze and diplopia. Findings include proptosis, lid retraction, periorbital edema, conjunctival injection, and exposure keratitis. Eye disease can precede, accompany, or follow the hyperthyroidism.

Risk and course. Smoking is the strongest modifiable risk factor for onset, severity, and poor treatment response; radioiodine, uncontrolled thyroid function, and high TRAb also contribute. An active inflammatory phase (pain, redness, swelling; Clinical Activity Score ≥3 of 7) lasts months before burning out into inactive fibrosis.

Teprotumumab. A monoclonal antibody against the IGF-1 receptor that reduces proptosis and diplopia. Adverse effects include hyperglycemia, hearing loss, muscle cramps, flares of inflammatory bowel disease, and teratogenicity, so contraception is required.

Reduced acuity, color desaturation, a relative afferent pupillary defect, or field loss signals dysthyroid optic neuropathy from muscles crowding the orbital apex. Give high-dose IV methylprednisolone immediately and proceed to decompression if vision does not recover.