Subclinical hyperthyroidism

Today we discussed subclinical hyperthyroidism

This is defined as
1) Low (usually undetectable) TSH
2) Normal T3 and T4
3) no symptoms of hyperthyroidism

The causes of subclinical hyperthyroidism mirror the causes of overt hyperthyroidism:
1) Exogenous T4
2) Toxic adenoma or multinodular goiter
3) Subacute thyroiditis
4) Graves' disease
5) Amiodarone
Others...

Differential of a low TSH with normal T3/T4 besides subclinical hyperthyroidism:
1) "Sick euthyroid"- stress causing decreased peripheral conversion to T3 (i.e. low), normal T4, low TSH.
2) Glucocorticoids, dopamine
3) Autonomous thyroid nodule producing enough T3/T4 to suppress TSH but not cause high T3/T4 levels

Workup consists of same as overt hyperthyroidism: radioactive iodine uptake and scan, possibly thyroid-stimulating antibodies.

Concerns in subclinical hyperthyroidism
1) Atrial fibrillation
2) Osteoporosis
3) Risk of progression to overt hyperthyroidism
4) Neuropsychiatric effects, diastolic dysfunction (softer evidence)

Natural history: Depends on underlying cause; ~2-5%/yr progress to overt hyperthyroidism

Whom to treat?

Controversial! Very little evidence.

This structured review from JAMA suggests considering treatment for patients who are older than 60 years and for those with or at increased risk for heart disease, osteopenia, or osteoporosis, or for those with subclinical Graves disease or nodular thyroid disease

Therapy consists of ablative therapy (i.e. radioactive iodine) with replacement or medications (PTU or methimazole) with goal to normalize TSH.

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