Underactive Thyroid: Symptoms, Levothyroxine Timing, and Monitoring

What is hypothyroidism and why does it develop?

Hypothyroidism means the thyroid gland does not produce enough of its hormones T4 and T3, slowing metabolism throughout the body. The most common cause worldwide is Hashimoto thyroiditis, an autoimmune condition in which the immune system mistakenly attacks and progressively destroys thyroid tissue over months to years. A comprehensive review published in JAMA in 2025 estimated that two to five percent of the general adult population in most societies is affected, with women developing it seven times more frequently than men.

Other causes include surgical removal of the thyroid gland for cancer or other thyroid disease, radioactive iodine treatment for an overactive thyroid, and medication effects from drugs including amiodarone (used for heart rhythm disorders) and lithium (used in bipolar disorder). Iodine deficiency remains a global cause but is much less common in most Arab countries now that iodine is routinely added to table salt.

Hashimoto thyroiditis deserves specific mention because it is responsible for the great majority of hypothyroid cases in countries without iodine deficiency. A landmark clinical review by Caturegli and colleagues in Autoimmunity Reviews in 2014 described the combination of raised anti-TPO antibodies and a characteristic ultrasound pattern that together characterise the condition. Understanding the autoimmune basis explains why there is no treatment that stops the underlying process: only thyroid hormone replacement manages the consequences.

What does hypothyroidism feel like? Why is it so often missed?

Hypothyroid symptoms develop so gradually that they are frequently attributed to ageing, overwork, or stress rather than a treatable medical condition. Persistent fatigue that does not improve with rest, unexplained weight gain despite unchanged diet, cold intolerance that others in the same room do not share, and constipation that appears without any change in diet are the central symptoms. A slow heart rate and rising cholesterol are often the findings that prompt initial investigation.

Skin and hair changes are characteristic. Dry, coarse skin, hair that becomes brittle and falls more than usual, and thinning of the outer third of the eyebrows are specific signs of prolonged untreated hypothyroidism. Swelling around the eyes and face from accumulation of water-retaining substances in the subcutaneous tissue is another classical finding. Depression and impaired concentration improve significantly in most people once TSH is normalised, which suggests the brain is directly affected by thyroid hormone deficiency.

Subclinical hypothyroidism, where TSH is elevated but free T4 remains within the normal range, is even more frequently missed because symptoms are minimal or absent. This is a common finding on routine blood testing and the decision to treat it with medication depends on the TSH level, the presence of symptoms, the patient's age, cardiovascular risk, and pregnancy planning. It should not be dismissed without a conversation about the specific circumstances.

How is hypothyroidism diagnosed?

Diagnosis begins with a TSH blood test. When the thyroid gland is underperforming, the pituitary gland raises TSH output to try to stimulate more hormone production. A raised TSH is therefore the earliest and most sensitive sign of hypothyroidism, appearing before T4 levels fall out of range. The generally accepted normal range is 0.4 to 4.0 mIU/L, though laboratories use slightly different reference intervals depending on the assay and population they are calibrated against.

When TSH is elevated, a free T4 measurement is added to determine whether the hypothyroidism is overt (high TSH with low free T4, always requiring treatment) or subclinical (high TSH with normal free T4, where the treatment decision is individualised). Anti-TPO antibody testing is done when Hashimoto thyroiditis is suspected, and a raised titre confirms the autoimmune basis and signals an increased risk of progressing to overt hypothyroidism over time.

A practical point relevant to the region: TSH results from different laboratories can vary modestly because of differences in assay calibration. If your result sits at the borderline of normal with symptoms present, repeating the test in an accredited laboratory is worthwhile before making treatment decisions. A single result is less informative than a trend observed across two to three measurements over weeks.

How to take levothyroxine in the way that makes treatment work

Levothyroxine is a synthetic form of T4 that replaces what the failing thyroid cannot produce. It is the standard treatment for hypothyroidism worldwide and is very effective when absorbed properly. A 2025 paper in Medicina specifically addressing levothyroxine malabsorption identified suboptimal intake timing as one of the most common causes of persistently high TSH in patients who report taking their medication correctly.

The rule is to take levothyroxine on an empty stomach, meaning at least 30 to 60 minutes before eating or drinking anything other than water. Coffee, even black without milk or sugar, significantly reduces absorption. In a region where coffee and tea are the first thing many people reach for in the morning, this point explains why TSH fails to reach target despite apparent adherence. Taking the tablet when waking and waiting before the morning drink is the simplest reliable way to apply this consistently.

The dose is set based on body weight and response, and adjusted gradually over weeks. The goal is to bring TSH into the normal range, not to suppress it to zero. Starting at too high a dose to accelerate response causes symptoms of thyroid overactivity, including palpitations, anxiety, and poor sleep, and increases bone loss over time. A gradual titration to the correct dose is always better than starting high.

What interferes with levothyroxine absorption?

Several common supplements and medications interfere with levothyroxine absorption in the gut. Oral iron supplements, commonly taken by many women in the region for anaemia, bind to levothyroxine in the intestine and reduce its absorption substantially. The solution is a minimum four-hour separation, taking levothyroxine in the morning and iron in the evening, or vice versa as long as the gap is maintained.

Calcium supplements, whether taken as bone protection or in antacids, interfere similarly. This includes combination calcium-vitamin D preparations which are widely used in the region. Vitamin D alone does not interfere, but the calcium component in combination products does. Aluminium- and magnesium-containing antacids also reduce absorption. The practical recommendation is to separate all of these from levothyroxine by at least four hours.

People with conditions that reduce gastrointestinal absorption in general, including coeliac disease, atrophic gastritis, and after certain bariatric surgeries, often need higher levothyroxine doses than their body weight would predict. This is why comparing your dose to that of a family member taking the same medication is rarely informative: absorption efficiency varies significantly between individuals based on the condition of their gut.

How often should TSH be checked? A monitoring schedule

TSH should be rechecked 6 to 8 weeks after starting levothyroxine or after any dose change. The reason for this interval is that TSH takes weeks to reach its new steady state after a dose adjustment. Checking TSH two weeks after a dose change gives a result that does not yet reflect the true equilibrium and will mislead the decision about whether to adjust further.

Once the dose is stable and TSH is in the target range, annual testing is appropriate for most adults. Situations that require earlier retesting include pregnancy, a weight change of five kilograms or more, adding or stopping any medication known to interfere with absorption, and any significant change in symptoms. When symptoms return or new ones appear, do not wait for the annual check.

For consistency, try to have TSH measured at the same laboratory and under the same conditions each time. If you normally take levothyroxine in the morning and the test is done that day, take the tablet after the blood draw rather than before, to avoid a transient elevation in T4 that can temporarily suppress TSH and make the result unrepresentative.

Hypothyroidism and pregnancy: why the dose changes immediately

Pregnancy significantly increases thyroid hormone requirements, because the developing fetus depends entirely on maternal thyroid hormones in the first trimester before its own thyroid begins functioning. Women with known hypothyroidism typically need a 25 to 30 percent increase in their levothyroxine dose from the moment pregnancy is confirmed.

The THERAPY trial published in the Journal of Clinical Endocrinology and Metabolism in 2010 demonstrated that proactive dose increase at the time of a positive pregnancy test, before waiting for TSH results, leads to better thyroid control and pregnancy outcomes than waiting to retest. This is why current guidance in most countries advises women with hypothyroidism to contact their doctor as soon as they see a positive home pregnancy test, not at the first antenatal visit weeks later.

After delivery, requirements return to pre-pregnancy levels in most women, and the pre-pregnancy dose is typically resumed. A TSH check at six weeks postpartum confirms this. Women diagnosed with hypothyroidism for the first time during pregnancy, which is detected on routine antenatal screening, require careful evaluation postpartum to determine whether lifelong treatment is needed or whether the condition was transient.

Living with hypothyroidism: what to expect long term

Most people with well-treated hypothyroidism live entirely normally without restriction. The medication itself carries no serious long-term risks at the doses used to normalise thyroid function. The principal inconvenience is remembering to take the tablet correctly every morning and organising the timing around coffee and supplements. After these habits are established, the condition requires little active management beyond the annual blood test.

Hair loss often worries people newly diagnosed with hypothyroidism. It typically improves substantially within three to six months of TSH normalisation, though it can temporarily worsen during the initial treatment phase before improving. Skin dryness and cold intolerance usually resolve more quickly, within the first few weeks of effective treatment, as metabolism begins to normalise.

If symptoms persist despite a TSH in the normal range, other causes should be considered. Anaemia, vitamin D deficiency, coeliac disease, depression, and sleep apnoea all cause overlapping symptoms and are more common in people with autoimmune thyroid disease than in the general population. Normalising TSH is the starting point, but it should not be the end of the investigation if significant symptoms remain.

Sources

• Wilson SA, Stem LA, Bruehlman RD et al: Hypothyroidism: Diagnosis and Treatment, American family physician, 2021
• Chaker L, Papaleontiou M: Hypothyroidism: A Review, JAMA, 2025
• Klubo-Gwiezdzinska J, Wartofsky L: Hashimoto thyroiditis: an evidence-based guide to etiology, diagnosis and treatment, Polish archives of internal medicine, 2022
• Mana DL, Rizzo LFL: Challenges in the treatment of hypothyroidism: malabsorption and pseudo-malabsorption of levothyroxine, Medicina, 2025
• Yassa L, Marqusee E, Fawcett R et al: Thyroid hormone early adjustment in pregnancy (the THERAPY) trial, The Journal of clinical endocrinology and metabolism, 2010
• Caturegli P, De Remigis A, Rose NR et al: Hashimoto thyroiditis: clinical and diagnostic criteria, Autoimmunity reviews, 2014