Reference Cards
Lab Differential Review
TFTs, Thyroid Function Tests

TSH, Throid Stimulation Hormone (produced and secreted by the anterior pituitary via negative feedback via free T3 and free T4); Normal Range: 0.3-5.0 mIU/I; May help monitor thyroid hormone replacement Rx. Ultrasensitive assays are available.
Decreased TSH: primary hyperthyroidism, thyrotoxicosis, secondary (pituitary - TSH deficiency - panhypopituitadsm) or tertiary (hypothalamic - infection - malignant infilitration - sarcoid) hypothyroidism, high dose steroids, signif. acute illness, subdinical hyperthyroidism
Increased TSH: hypothyroidism (S/P radiation - radioiodine - surgery, idiopathic, congenital, goiter [maternal origin, medication - lithium - aminosalicylic acid - phenylbutazone, lack of iodine, Hashimoto's]); If follow up FT4 is normal, check TRH test for incipient hypothryoidism.

Free T4, FT4: free T4 is a small percentage of the total T4; free T4 is the biologically active component
Increased FT4: hyperthyroidism, thyrotoxicosis (Graves'disease [autoimmune disorder] - diffuse toxic goiter, solitary toxic adenoma, toxic multinodular goiter, cancer, subacute and chronic thyroiditis, struma ovarii), exogenous T4 - thyrotoxicosis factitia - factitious hyperthyroidism, large beta blocker dose, amiodarone
Decreased FT4: hypothyroidism

Total T4, Thyroxine: Normal Range: 5.0-12.0 ug/dl
Increased T4: hyperthyroidism, acute thyroiditis, increased TBG (thyroid - thyroxine binding globulin), pregnancy, contrast media with iodine, meds (iodine, amphethamines, oral contraceptives, estrogens, amiodarone, large beta blocker dose)
Decreased T4: hypothyroidism, myxedema, chronic and subacute thyroiditis, cretinism, nephrosis, chronic liver pathology, low TBG, meds (PTU or other antithyroid med, NSAIDs, lithium, phenytoin, carbamazepine, anabolic steroids), iodine deficiency

Thyroxine or Thyroid Binding Globulin, TBG
Increased TBG: pregnancy, estrogen, ERT (estrogen replacement therapy), viral hepatitis
Decreased TBG: malnutrition, nephrotic syndrome, hypoproteinemia, protein losing enteropathy, menopause, stress

Triiodothyronine, T3 - 8-10% of thyroid hormone, 70% is protein bound, may be increased with normal T4 levels with clinical thyrotoxicosis
Increased T3 - thyrotoxicosis, excess po T4, excess TBG
Decreased T4 - significant hypothyroidism, TBG deficiency, other severe illness

24 Hour I-131 Uptake with Hyperthyroidism: Increased: Graves', toxic adenoma, toxic multinodular goiter, thyroid CA, TH resistance, TSH secreting pituitary adenoma
Decreased: thyroiditis, excess po TH, ectopic secreting thyroid tissue (struma ovarii, thyroid CA mets), thyrotoxicosis via iodine administration, Graves' S/P iodine Rx