6
Table 4. Rarer Causes of Perturbation of the Hypothalamic-
Pituitary-Thyroid Axis
Axis perturbation Prevalence Mechanism
Secondary or tertiary
(central) hypothyroidism
Central congenital
hypothyroidism 1:13–16,000
Proportion of hypothyroid
patients 1:1,000
Postpartum 1:20,000
Pituitary/hypothalamic
Autonomous TSH secretion 1:1,000,000 TSH-secreting pituitary
adenomas
TSH insensitivity syndromes Not available Failure of TSH to stimulate
thyroid
TH resistance 1:19,000 Genetic variation in TH
receptors
Table 5. Clinical Utility of Triiodothyronine Measurement
• Total T3 measurement is preferred over free T3 measurement.
• Total T3 measurement may be useful in the evaluation of patients with suppressed TSH
levels.
• Total T3 measurement is not helpful in the evaluation of patients with suspected
hypothyroidism.
• Total T3 measurement can be helpful in:
▶ the management of patients on T4 and T3 combination therapy for the treatment
of hypothyroidism,
▶ the monitoring of patients on suppressive doses of T4 for treatment of thyroid
cancer and
▶ the evaluation of patients on T4 treatment for hypothyroidism with suspected T4
to T3 conversion defects.
• Management of patients with Graves' disease, as alterations of T3/T4 ratio can be
helpful in identifying patients whose disease may remit.
• Reverse T3 (rT3) measurement is rarely useful, two very rare exceptions are the diagnosis of:
▶ infrequent genetic thyroid syndromes and
▶ consumptive hypothyroidism, a complication of unusual pediatric and adult tumors.
