Thyroid Physiology and Management Overview
The thyroid gland produces thyroid hormones (T4 and T3) that regulate metabolism in virtually every body system, with TSH being the primary screening test for thyroid dysfunction in most clinical settings 1.
Key Physiological Principles
Thyroid Hormone Binding and Measurement
- Approximately 99.97% of T4 and 99.7% of T3 are protein-bound (primarily to thyroid-binding globulin/TBG)
- Free hormone represents the metabolically active fraction
- In pregnancy, total T4 measurement is preferred over free T4 direct immunoassay because alterations in serum proteins cause free T4 assays to yield falsely low values 2
- During pregnancy, total T4 and T3 reference ranges increase to approximately 1.5 times the nonpregnant range from late first trimester onward 3, 2
Normal Reference Ranges
The TSH reference range has an upper limit of 4.12 mIU/L based on NHANES III data, with mean/median values around 1.5 mIU/L 2. However, this remains controversial as some advocate for an upper limit of 2.5-3.0 mIU/L. For clinical practice, use the standard reference range of 0.45-4.5 mIU/L for TSH in non-pregnant adults 4.
Critical caveat: TSH values vary diurnally by up to 50% and can fluctuate 40% even when measured at the same time of day 2. This physiologic variation must be considered before diagnosing thyroid dysfunction.
Hypothyroidism Management
Diagnosis
Primary hypothyroidism is diagnosed by elevated TSH with low free T4; subclinical hypothyroidism shows elevated TSH with normal free T4 2, 5.
Treatment Approach
Levothyroxine monotherapy is the standard treatment for hypothyroidism 5, 6. Key management principles:
- Take levothyroxine with water 30-60 minutes before breakfast or at bedtime 4 hours after the last meal 2
- Blood for free T4 assessment should be collected before dosing (levels transiently increase up to 20% post-dose, peaking at 3.5 hours) 2
- Target TSH within the normal reference range for non-pregnant patients 2
- No evidence supports targeting specific TSH values within the normal range in non-pregnant patients 2
Subclinical Hypothyroidism
In older adults, treat subclinical hypothyroidism only if TSH ≥7-10 mIU/L, as observational data show increased cardiovascular risk at these levels 7. For TSH <7 mIU/L, treatment is not supported by evidence in older adults 7.
Pregnancy-Specific Management
For pregnant women on levothyroxine, measure TSH promptly after conception and adjust dosage with these trimester-specific targets 2:
- First trimester: TSH <2.5 mIU/L
- Second trimester: TSH <3.0 mIU/L
- Third trimester: TSH <3.5 mIU/L
Monitor TSH every 4 weeks during the first half of pregnancy and at least once between 26-32 weeks gestation 2.
Common Pitfalls
- Overtreatment occurs in 20% of patients and causes cardiovascular complications and accelerated bone loss, particularly in elderly and postmenopausal women 2
- High-dose levothyroxine (>200 µg/d) with persistently elevated TSH suggests non-compliance (most common) or absorption problems 2
- Patients with adrenal insufficiency must receive hydrocortisone before thyroid hormone replacement to avoid adrenal crisis 2
Hyperthyroidism Management
Diagnosis in Pregnancy
Diagnose hyperthyroidism in pregnancy using suppressed TSH with either: (1) total T4 and T3 adjusted to 1.5 times nonpregnant range, or (2) free T4 and free T3 with trimester-specific reference ranges 3.
Critical distinction: Low TSH with normal free T4 in early pregnancy represents physiologic hCG-mediated TSH suppression and does NOT require treatment 3. This occurs in the first half of normal pregnancy due to hCG stimulation of the thyroid.
Pregnancy Treatment Algorithm
For Graves' disease requiring treatment in pregnancy 3:
- First trimester: Use propylthiouracil (PTU)
- After first trimester: Switch to methimazole
This trimester-specific approach balances the rare but documented teratogenic effects of methimazole against PTU's hepatotoxicity risk.
Subclinical Hyperthyroidism
Treat subclinical hyperthyroidism with TSH <0.1 mIU/L in older adults due to increased cardiovascular risk and bone density loss 7. Long-term low-dose methimazole is a viable alternative to radioactive iodine in older adults 7.
When to Consult Endocrinology
Refer to endocrinology for 2:
- Pregnancy or women planning conception
- Difficulty achieving/maintaining euthyroid state
- Presence of goiter, nodules, or structural thyroid changes
- Cardiac disease
- Other endocrine disorders (adrenal, pituitary)
- Unusual thyroid function test patterns
- Infants and children
Key Laboratory Considerations
In hospitalized patients, interpret thyroid function tests cautiously as non-thyroidal illness syndrome commonly causes transient abnormalities that mimic thyroid disease without actual thyroid pathology 8. Avoid routine TSH screening in acutely ill hospitalized patients unless there is specific clinical suspicion for thyroid dysfunction.
Age- and sex-specific reference ranges are increasingly recognized as important for accurate diagnosis, as standard reference ranges may lead to misclassification of thyroid status 9.