The Thyroid Gland: Function and Disorder Management
Role of the Thyroid Gland
The thyroid is a butterfly-shaped endocrine gland located in the anterior neck that produces thyroid hormones (T4 and T3), which regulate basal metabolism, oxygen consumption, growth, brain function, and development across nearly all body tissues. 1
Primary Functions
Hormone Production: The thyroid synthesizes and secretes thyroxine (T4) and triiodothyronine (T3), which influence metabolic rate, cardiovascular function, bone metabolism, reproductive function, cognitive function, gastrointestinal function, and glucose/lipid metabolism 2, 1
Metabolic Regulation: Thyroid hormones control basal metabolic processes and enhance oxygen consumption in nearly all body tissues, with major target organs including brain, liver, muscle, heart, and adipose tissue 1, 3
Growth and Development: These hormones influence linear growth, neural development, brain function (including intelligence and memory), dentition, and bone development 1
Hormone Synthesis Pathway
TSH Regulation: Thyroid hormone production is primarily regulated by thyroid-stimulating hormone (TSH) from the pituitary gland, which controls iodide uptake, activates thyroid peroxidase enzyme, regulates iodide oxidation and organification, controls T4/T3 secretion, and influences thyroglobulin modifications 4, 5
Iodine Requirement: Adequate iodine intake is essential for proper thyroid hormone synthesis, with daily requirements of 150 μg/day in adults 4, 6
Cofactor Dependencies: Iron and selenium are crucial for proper thyroid function—iron deficiency impairs thyroid metabolism, while selenium is required for deiodination of T4 to T3 via selenoenzyme Type 1 5'-deiodinase 4, 6
Thyroid Disorders and Treatment
Hypothyroidism
Hypothyroidism represents thyroid hormone insufficiency resulting in decreased metabolism, with primary hypothyroidism typically caused by autoimmune thyroiditis or iodine deficiency, diagnosed by elevated TSH levels. 7
Diagnosis
- Primary Hypothyroidism: Characterized by elevated TSH and low free T4 levels 4
- TSH Screening: TSH is the most sensitive test for thyroid dysfunction, with sensitivity above 98% and specificity greater than 92% 8, 4
- Confirmation Testing: Measure both TSH and free T4 to distinguish between subclinical hypothyroidism (normal free T4) and overt hypothyroidism (low free T4) 8
Treatment with Levothyroxine
First-Line Therapy: Levothyroxine (T4) is the treatment of choice for hypothyroidism 2, 7
Dosing for Adults <70 Years: Full replacement dose of approximately 1.6 mcg/kg/day for patients without cardiac disease or multiple comorbidities 8, 2
Dosing for Elderly/Cardiac Patients: Start with lower dose of 25-50 mcg/day and titrate gradually for patients >70 years or with cardiac disease to avoid cardiac decompensation, angina, or arrhythmias 8, 2
Administration: Administer once daily on an empty stomach, one-half to one hour before breakfast with a full glass of water 2
Monitoring: Check TSH every 6-8 weeks while titrating hormone replacement; once stable, repeat testing every 6-12 months 8, 2
Target TSH: Maintain TSH within reference range (0.5-4.5 mIU/L) with normal free T4 levels 8
Critical Safety Considerations
Adrenal Insufficiency: Before initiating levothyroxine, rule out concurrent adrenal insufficiency, as starting thyroid hormone before corticosteroids can precipitate life-threatening adrenal crisis 8, 2
Narrow Therapeutic Index: Levothyroxine has a narrow therapeutic index—overtreatment increases risk for atrial fibrillation, osteoporosis, fractures, abnormal cardiac output, and ventricular hypertrophy, while undertreatment adversely affects growth, development, cardiovascular function, and metabolism 2
Pregnancy: Levothyroxine requirements increase 25-50% during pregnancy; measure TSH and free T4 as soon as pregnancy is confirmed and monitor every 4 weeks until stable 2
Hyperthyroidism
Hyperthyroidism represents thyroid hormone excess that increases metabolic rate and causes symptoms including anxiety, tremor, tachycardia, and weight loss, with Graves disease being the most common etiology in developed countries. 7
Diagnosis
- Laboratory Findings: Characterized by low TSH and elevated free T4 levels 4
- Evaluation: Assess with TSH and free thyroxine level measurement 7
Treatment Options
- Beta-Blockers: For symptomatic relief of tachycardia, tremor, and anxiety 4
- Antithyroid Medications: Methimazole or propylthiouracil to block hormone synthesis 4, 7
- Radioactive Iodine: Definitive treatment option for many patients 4, 7
- Surgery: Thyroidectomy for selected cases 4, 7
Thyroid Nodules
- Evaluation: Patients with thyroid nodules should undergo TSH level measurement and thyroid ultrasonography to guide further testing 7
- Clinical Significance: Nodules may or may not be associated with thyroid hormone abnormalities 7
Common Pitfalls in Thyroid Management
Overtreatment: Approximately 25% of patients on levothyroxine are unintentionally maintained on doses sufficient to fully suppress TSH, increasing risks for cardiac complications and osteoporosis 8
Single TSH Value: Do not treat based on a single elevated TSH value, as 30-60% normalize on repeat testing 8
Drug Interactions: Administer levothyroxine at least 4 hours before or after drugs that interfere with absorption 2
Transient Thyroiditis: Failure to recognize transient hypothyroidism may lead to unnecessary lifelong treatment 8