# Hypothyroidism

Use TSH and free T4 to distinguish primary from central hypothyroidism, identify myxedema coma immediately, individualize levothyroxine initiation for cardiovascular risk, and avoid routine treatment of mild persistent subclinical disease in older adults.

**Clinical question:** How should physicians diagnose, treat, and monitor overt, subclinical, central, and severe hypothyroidism?

Updated: 2026-08-21T02:29:53.661696+00:00

## What matters in practice
- For suspected primary hypothyroidism, serum TSH is the best initial diagnostic test; elevated TSH with low free T4 establishes overt primary hypothyroidism. [2][4]
- Do not use TSH alone when central hypothyroidism is possible: low free T4 with an inappropriately low or normal TSH suggests central TSH deficiency. [2]
- In uncomplicated adults with newly diagnosed overt hypothyroidism, levothyroxine 1.6 mcg/kg/day is a typical full-replacement starting dose; start 25 mcg/day in adults older than 65 years or with heart disease. [17]
- Recheck TSH about 6-8 weeks after starting or changing levothyroxine; once stable, reassess at 4-6 months and then annually. [17][19]
- Treat suspected myxedema coma in the ICU without waiting for thyroid results: administer stress-dose IV glucocorticoid before IV levothyroxine. [18][21]
- For persistent subclinical hypothyroidism in older adults, do not routinely initiate levothyroxine unless TSH reaches 10 mIU/L or overt primary hypothyroidism develops. [23]

## Interpret TSH and free T4 before selecting treatment

The initial laboratory pattern determines whether TSH can guide both diagnosis and dose titration.

Order serum TSH as the principal diagnostic test when primary thyroid failure is suspected. An elevated TSH with low free T4 indicates overt primary hypothyroidism; elevated TSH with free T4 in the laboratory reference interval indicates subclinical hypothyroidism. [2][4][12]

Obtain free T4 whenever TSH is abnormal and whenever pituitary or hypothalamic disease is clinically plausible. Low free T4 accompanied by a low or non-elevated TSH is discordant with primary gland failure and suggests central TSH deficiency; monitor replacement with free T4 rather than TSH in secondary or tertiary hypothyroidism. [2][17]

Interpret a mildly elevated TSH against age and persistence. TSH shifts upward with age in people without demonstrable thyroid disease, and subclinical hypothyroidism becomes more prevalent in later life. In the major older-adult replacement trial, eligibility required TSH 4.6-19.9 mU/L, normal free T4, and confirmation on at least two measurements separated by at least 3 months; this approach helps avoid treating a transient abnormality. [13][15]
- Overt primary hypothyroidism: high TSH plus low free T4; initiate replacement unless a reversible transient context changes the assessment. [4]
- Subclinical hypothyroidism: high TSH plus normal free T4; confirm persistence before long-term treatment decisions, particularly in adults 65 years and older. [12][13]
- Central hypothyroidism: low free T4 with low or inappropriately normal TSH; use free T4, not TSH, to assess treatment adequacy. [2][17]

*Thyroid-function patterns that change the diagnostic and monitoring pathway. [2][4][12][17]*

| Laboratory pattern | Interpretation | Next action |
| --- | --- | --- |
| High TSH; low free T4 | Overt primary hypothyroidism. [4] | Start levothyroxine and titrate to TSH after steady state. [17][19] |
| High TSH; normal free T4 | Subclinical hypothyroidism. [12] | Confirm persistence; in older adults, generally defer treatment unless TSH is at least 10 mIU/L or free T4 becomes low. [13][23] |
| Low free T4; low or normal TSH | Central TSH deficiency is suggested. [2] | Assess and dose-monitor with free T4 rather than TSH. [17] |

## Start levothyroxine according to cardiac risk and biochemical severity

Levothyroxine is the standard replacement agent; initial dose should reflect age and cardiovascular vulnerability.

For adults with newly diagnosed hypothyroidism and no complicating cardiac disease, an initial levothyroxine dose of 1.6 mcg/kg/day is a typical full-replacement strategy. In patients older than 65 years or those with pre-existing heart disease, begin at 25 mcg/day and increase by 12.5-25 mcg every 4-6 weeks rather than immediately using a full weight-based dose. [17]

Use TSH to titrate primary hypothyroidism, obtaining the first post-initiation or post-adjustment measurement after approximately 6-8 weeks. After the correct dose is achieved, repeat testing at 4-6 months and then every 12 months. Recheck after a formulation or brand change and after starting or stopping medications that affect thyroid hormone levels. [17][19][24]

A persistently abnormal TSH while prescribed levothyroxine should prompt a structured medication review before repeated empiric dose escalation. Verify the product and formulation, interval adherence, food and medication interactions, and new therapies that alter thyroid hormone levels; fluctuating TSH or an unusually high dose requirement warrants this review. [24]

Use free T4 rather than TSH to monitor central hypothyroidism. In hospitalized patients unable to take oral thyroid replacement, IV levothyroxine may be used at approximately 50%-75% of the oral dose; reserve IV treatment for strict inability to use the enteral route or myxedema coma. [17][24]
- Uncomplicated adult overt hypothyroidism: levothyroxine 1.6 mcg/kg/day. [17]
- Age older than 65 years or established heart disease: levothyroxine 25 mcg/day, then increase by 12.5-25 mcg every 4-6 weeks. [17]
- Primary hypothyroidism monitoring: TSH at 6-8 weeks after initiation or dose change, then 4-6 months after stabilization and annually thereafter. [17][19]
- Central hypothyroidism monitoring: follow free T4, not TSH. [17]

### Pregnancy

Pregnancy increases levothyroxine requirements; monitor TSH and adjust dose during gestation. For newly diagnosed maternal hypothyroidism, a cited initial levothyroxine dose is 1.8 mcg/kg/day with dose adjustment every 4 weeks as needed. Use levothyroxine rather than T3-containing or desiccated thyroid preparations in pregnancy. [19]
- Known hypothyroidism entering pregnancy: promptly reassess TSH and adjust levothyroxine because requirements may rise. [19]
- Newly diagnosed hypothyroidism during pregnancy: initiate levothyroxine 1.8 mcg/kg/day and reassess every 4 weeks for dose adjustment. [19]

*Initial levothyroxine selection and follow-up. [17][19][24]*

| Clinical setting | Initial strategy | Laboratory follow-up |
| --- | --- | --- |
| Adult without cardiac complications | Levothyroxine 1.6 mcg/kg/day. [17] | TSH in 6-8 weeks after initiation or dose modification. [17][19] |
| Age >65 years or pre-existing heart disease | Levothyroxine 25 mcg/day; increase by 12.5-25 mcg every 4-6 weeks. [17] | Use serial TSH to guide gradual titration. [17] |
| Central hypothyroidism | Individualize levothyroxine replacement. [17] | Assess free T4 rather than TSH. [17] |
| Pregnancy with newly diagnosed hypothyroidism | Levothyroxine 1.8 mcg/kg/day. [19] | Check TSH and adjust every 4 weeks as needed. [19] |

## Reserve treatment of subclinical hypothyroidism in older adults for clear biochemical progression

Age, persistence, free T4, and treatment harms determine whether a raised TSH warrants replacement.

In older adults, persistent subclinical hypothyroidism should not automatically trigger levothyroxine. A practical threshold is to withhold treatment unless TSH increases to at least 10 mIU/L or overt primary hypothyroidism emerges, defined by elevated TSH with low free T4. [23]

When treatment is chosen for an older patient, use a start-low, go-slow approach: levothyroxine 25-50 mcg daily with increases every 6-8 weeks. The cited age-specific targets are TSH below 6 mIU/L for adults younger than 80 years and below 7 mIU/L for adults 80 years or older, while avoiding angina or other evidence of excessive replacement. [23]

This conservative strategy is especially relevant because TSH increases progressively with age in otherwise euthyroid people and because subclinical hypothyroidism is common after age 65. Confirm elevated TSH with normal free T4 on repeat testing before labeling persistent disease or committing a patient to long-term replacement. [13][15]
- Older adult with TSH below 10 mIU/L and normal free T4: generally observe after confirming persistence. [23]
- TSH at least 10 mIU/L or conversion to low free T4: consider levothyroxine initiation. [23]
- If treating an older adult: start 25-50 mcg/day and titrate every 6-8 weeks, with attention to angina. [23]

*Decision framework for persistent subclinical hypothyroidism in older adults. [13][23]*

| Finding | Management implication | Dose and target if treated |
| --- | --- | --- |
| Elevated TSH with normal free T4 not yet confirmed | Repeat thyroid testing; trial enrollment definitions required two abnormal results at least 3 months apart. [13] | Do not commit to chronic replacement before persistence is established. [13] |
| Persistent TSH <10 mIU/L with normal free T4 | Do not routinely initiate levothyroxine. [23] | Continue biochemical surveillance. [23] |
| TSH ≥10 mIU/L or elevated TSH with low free T4 | Initiate treatment or treat as overt primary hypothyroidism, respectively. [23] | For older adults, start 25-50 mcg/day; adjust every 6-8 weeks. [23] |

## Treat suspected myxedema coma before confirmatory results return

Altered mental status with hypothermia and multisystem decompensation requires ICU-level treatment.

Suspect myxedema coma in a patient with hypothyroidism and altered mental status, particularly with hypothermia, bradycardia, hyponatremia, heart failure, or hypopnea. Common precipitants include surgery, infection, cold exposure, and sedative administration. The diagnosis is clinical; treatment should begin when suspicion is high rather than awaiting thyroid results. [21][22]

Draw random cortisol, TSH, free T4, and free T3 if this does not delay therapy. Give IV hydrocortisone before thyroid hormone because thyroid replacement can increase cortisol metabolism and unmask adrenal insufficiency; a cited regimen is hydrocortisone 100 mg IV initially, totaling 200-400 mg/day, with de-escalation guided by cortisol results and resolution of hypotension. [18][21]

Administer IV levothyroxine with a 200-400 mcg loading dose, using lower doses in smaller or older patients and those with coronary disease or arrhythmia. Follow with replacement at 1.6 mcg/kg/day, reduced to 75% while administered intravenously; transition to enteral therapy after clinical improvement. Measure TSH, free T4, and free T3 every 24-48 hours for dose adjustment during acute treatment. [18][21]

Admit to intensive care for cardiorespiratory support and active treatment of the precipitating illness. Reported mortality associated with myxedema coma has been as high as 80%, making prompt thyroid hormone replacement, glucocorticoid coverage, and physiologic support more important than diagnostic perfection. [21][22]
- Immediate labs: random cortisol, TSH, free T4, and free T3; do not delay treatment for results. [21]
- Glucocorticoid first: hydrocortisone 100 mg IV, then 200-400 mg/day pending cortisol assessment and hemodynamic recovery. [21]
- Thyroid hormone: IV levothyroxine 200-400 mcg loading dose, then 1.6 mcg/kg/day equivalent reduced to 75% while IV. [18]
- Monitoring: repeat TSH, free T4, and free T3 every 24-48 hours during acute dose adjustment. [21]

*Immediate management sequence for suspected myxedema coma. [18][21][22]*

| Sequence | Action | Decision point |
| --- | --- | --- |
| 1 | ICU admission and obtain cortisol, TSH, free T4, and free T3. [21] | Do not await results when clinical suspicion is high. [21] |
| 2 | Hydrocortisone 100 mg IV, followed by 200-400 mg/day. [21] | Administer before thyroid hormone; taper or stop based on cortisol and hemodynamic recovery. [21] |
| 3 | IV levothyroxine 200-400 mcg loading dose, then 75% of 1.6 mcg/kg/day while IV. [18] | Use lower loading doses for older adults and patients with coronary disease or arrhythmia. [18] |
| 4 | Repeat TSH, free T4, and free T3 every 24-48 hours. [21] | Adjust acute replacement and transition to enteral therapy after clinical improvement. [18][21] |

## Postpone elective surgery in severe hypothyroidism and recognize checkpoint inhibitor patterns

Severity and the treatment context determine whether replacement alone is adequate or urgent escalation is required.

Postpone nonemergent surgery in severe hypothyroidism, including myxedema coma, altered mentation, pericardial effusion, heart failure, or very low thyroxine below 1 mcg/dL. If surgery is emergent, rapidly replace thyroid hormone with IV levothyroxine 200-500 mcg loading followed by 50-100 mcg IV daily; consider IV liothyronine if myxedema coma is suspected and provide glucocorticoids when adrenal insufficiency is a concern. [22]

During immune checkpoint inhibitor therapy, hypothyroidism may follow a thyrotoxic phase. The biochemical definition remains elevated TSH with normal T4/T3 in subclinical disease or low T4/T3 in overt disease. In atezolizumab protocols, asymptomatic hypothyroidism was managed by continuing immunotherapy, initiating thyroid replacement, and checking TSH weekly; symptomatic hypothyroidism prompted temporary withholding until symptoms were controlled and thyroid function was improving. [10][11][14]
- Severe hypothyroidism before elective surgery: defer surgery until treated. [22]
- Emergent surgery with severe hypothyroidism: IV levothyroxine 200-500 mcg loading, then 50-100 mcg IV daily. [22]
- Atezolizumab-associated asymptomatic hypothyroidism: continue therapy, begin replacement, and monitor TSH weekly in the protocol setting. [10][11]
- Atezolizumab-associated symptomatic hypothyroidism: withhold treatment temporarily, initiate replacement, and resume after symptom control and improving thyroid function. [10][11]

## Common questions

### When should TSH not be used to monitor levothyroxine replacement?

Do not use TSH as the principal treatment marker in secondary or tertiary hypothyroidism. Low free T4 with low or non-elevated TSH suggests central TSH deficiency, and replacement should be assessed with free T4. [2][17]

## References
1. Thyroid Function Reference Intervals by Age, Sex, and Race — www.acpjournals.org — https://www.acpjournals.org/doi/10.7326/ANNALS-24-01559
2. Hypothyroidism — www.acpjournals.org — https://www.acpjournals.org/doi/pdf/10.7326/AITC202007070
3. Journal Pre-proof — www.cell.com — https://www.cell.com/iscience/pdf/S2589-0042(26)00397-4.pdf
4. Subclinical Hypothyroidism Is an Independent Risk Factor ... — www.acpjournals.org — https://www.acpjournals.org/doi/10.7326/0003-4819-132-4-200002150-00004
5. Hypothyroidism | Annals of Internal Medicine — www.acpjournals.org — https://www.acpjournals.org/doi/10.7326/AITC202007070
6. Thyroid Antibody Status, Subclinical Hypothyroidism, and the ... — academic.oup.com — https://academic.oup.com/jcem/article/99/9/3353/2538570
7. Pregnancy-specific Reference Intervals for TSH and FT4 — academic.oup.com — https://academic.oup.com/jcem/article/111/8/2341/8502205
8. Thyroid Dysfunction and Diabetes Mellitus - Oxford Academic — academic.oup.com — https://academic.oup.com/edrv/article/40/3/789/5288751
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10. OfficialTitle: APhaseIII,Open-Label,Multicenter,Three-Arm, ... — cdn.clinicaltrials.gov — https://cdn.clinicaltrials.gov/large-docs/79/NCT02788279/Prot_000.pdf
11. protocol — cdn.clinicaltrials.gov — https://cdn.clinicaltrials.gov/large-docs/42/NCT02409342/Prot_000.pdf
12. 3. HEALTH EFFECTS — www.atsdr.cdc.gov — https://www.atsdr.cdc.gov/toxprofiles/tp158-c3.pdf
13. Study Details | NCT01660126 | Thyroid Hormone Replacement for Subclinical Hypothyroidism | ClinicalTrials.gov — clinicaltrials.gov — https://clinicaltrials.gov/study/NCT01660126
14. Endocrine Dysfunction From Immune Checkpoint Inhibitors — ascopubs.org — https://ascopubs.org/doi/pdf/10.1200/OP.23.00023
15. The Thyroid Axis in Older Individuals with Persistent Subclinical — cdn.clinicaltrials.gov — https://cdn.clinicaltrials.gov/large-docs/75/NCT02399475/Prot_SAP_000.pdf
16. Study Details | NCT03606824 | Thyroid Hormone Replacement for Subclinical Hypothyroidism and Dyslipidemia in ASCVD (ThyroHeart-Lipid Study) | ClinicalTrials.gov — clinicaltrials.gov — https://clinicaltrials.gov/study/NCT03606824
17. Levothyroxine Treatment and the Risk of Cardiac Arrhythmias — pmc.ncbi.nlm.nih.gov — https://pmc.ncbi.nlm.nih.gov/articles/PMC8600254
18. Guidelines for the Treatment of Hypothyroidism: Prepared by the American Thyroid Association Task Force on Thyroid Hormone Replacement - PMC — pmc.ncbi.nlm.nih.gov — https://pmc.ncbi.nlm.nih.gov/articles/PMC4267409
19. Levothyroxine - StatPearls - NCBI Bookshelf - NIH — www.ncbi.nlm.nih.gov — https://www.ncbi.nlm.nih.gov/books/NBK539808
20. Initial treatment of myxedema coma using oral levothyroxine: a case report from Tanzania - PMC — pmc.ncbi.nlm.nih.gov — https://pmc.ncbi.nlm.nih.gov/articles/PMC9066563
21. Myxedema Coma - StatPearls - NCBI Bookshelf — www.ncbi.nlm.nih.gov — https://www.ncbi.nlm.nih.gov/books/NBK545193
22. Perioperative Management of Thyroid Dysfunction - PMC — pmc.ncbi.nlm.nih.gov — https://pmc.ncbi.nlm.nih.gov/articles/PMC5398303
23. Hypothyroidism in Older Adults - Endotext - NCBI Bookshelf — www.ncbi.nlm.nih.gov — https://www.ncbi.nlm.nih.gov/books/NBK279005
24. Hypothyroidism - StatPearls - NCBI Bookshelf — www.ncbi.nlm.nih.gov — https://www.ncbi.nlm.nih.gov/books/NBK519536

## Editorial note

Prepared from cited clinical literature using Astra's research workflow. Verify recommendations against current guidance and patient-specific factors.