Approach to Etiology in Nonpregnant Adults
A. Without nodular thyroid
Initial test: TRAb measurement (TSI or TBII assays).
o Positive → confirms Graves’ disease.
o Negative → does not rule out Graves; consider mild disease.
Next step if TRAb negative:
o Radioiodine uptake scan or thyroid Doppler (if expertise
exists).
B. With nodular thyroid
Initial test: Radioiodine uptake & scan to distinguish:
o Toxic multinodular goiter → multiple focal areas of uptake
o Toxic adenoma → single focal uptake
o Graves’ disease → diffuse uptake
Special Populations
Pregnancy:
o Radioiodine contraindicated.
o Use TRAb or ultrasound blood flow.
Breastfeeding:
o Iodine-123: discard milk for 5 days.
o Iodine-131: do not use during lactation.
Thyroid Function Test Patterns
Low TSH + high T3, normal T4 → Graves’ disease, toxic adenoma,
or exogenous T3.
Low TSH + high T4, normal T3 → exogenous T4, amiodarone-
induced, or nonthyroidal illness.
High/normal TSH + high T3/T4 → TSH-secreting pituitary adenoma
or thyroid hormone resistance.
A. Without nodular thyroid
Initial test: TRAb measurement (TSI or TBII assays).
o Positive → confirms Graves’ disease.
o Negative → does not rule out Graves; consider mild disease.
Next step if TRAb negative:
o Radioiodine uptake scan or thyroid Doppler (if expertise
exists).
B. With nodular thyroid
Initial test: Radioiodine uptake & scan to distinguish:
o Toxic multinodular goiter → multiple focal areas of uptake
o Toxic adenoma → single focal uptake
o Graves’ disease → diffuse uptake
Special Populations
Pregnancy:
o Radioiodine contraindicated.
o Use TRAb or ultrasound blood flow.
Breastfeeding:
o Iodine-123: discard milk for 5 days.
o Iodine-131: do not use during lactation.
Thyroid Function Test Patterns
Low TSH + high T3, normal T4 → Graves’ disease, toxic adenoma,
or exogenous T3.
Low TSH + high T4, normal T3 → exogenous T4, amiodarone-
induced, or nonthyroidal illness.
High/normal TSH + high T3/T4 → TSH-secreting pituitary adenoma
or thyroid hormone resistance.
