In males, the HPG axis governs testicular development, spermatogenesis, and testosterone production. These processes begin in foetal life, pause in childhood, and reactivate at puberty — shaping not only fertility, but muscle mass, libido, voice, and bone health.
The Hypothalamic–Pituitary–Gonadal (HPG) axis governs the development and regulation of reproductive physiology, including puberty, gametogenesis, menstrual cycles, and sexual function. It’s a dynamic system that responds to internal cues (like age and energy availability) and external signals (like stress and illness).
Its activity varies dramatically across the lifespan, from foetal differentiation to puberty, reproductive maturity, and eventual decline. Understanding this baseline physiology is essential before exploring disorders of puberty, fertility, or sexual function.
Interpreting an ECG isn’t just about spotting abnormalities — it’s about understanding the heart’s electrical story in the context of the patient. This guide breaks down the process step by step.
If you need a reminder of the electrophysiology of the heart you can find that here
Growth isn’t just about getting taller, it’s about coordinated development across tissues, organs, and systems. The hypothalamic-pituitary-somatomedin (HPS) axis governs this process, linking brain signals to liver output and peripheral tissue response. It’s a slow axis, but a powerful one - and when it fails, the consequences are lifelong.
The hypothalamic-pituitary-adrenal (HPA) axis is your body’s central stress-response system. It’s not just about cortisol, it’s about how the brain, pituitary, and adrenal glands coordinate to keep you alive under pressure. Whether you’re managing infection, trauma, or emotional strain, the HPA axis is quietly adjusting your metabolism, blood pressure, and immune response.
The thyroid axis is a finely tuned system that regulates metabolism, growth, and energy balance. Disturbances in this axis can present subtly or dramatically, and understanding them requires more than memorizing hormone levels, it demands clinical reasoning.
Hashimoto’s thyroiditis is the most common cause of hypothyroidism in iodine-sufficient regions — and a classic example of destructive autoimmunity. Unlike Graves’ disease, which stimulates the thyroid, Hashimoto’s gradually damages and disables it. The immune system mistakes thyroid tissue for a threat, and over time, hormone production declines.
In the last few posts, we explored what happens when thyroid hormone levels are too high and how Graves’ disease, toxic nodules, and thyroiditis each disrupt the HPT axis in different ways. Now we turn to the opposite problem: hypothyroidism.
What happens when there’s not enough thyroid hormone in the
body?
Unlike hyperthyroidism, where everything speeds up,
hypothyroidism causes a gradual slowing of metabolic processes across multiple
systems. Patients may feel tired, cold, constipated, and foggy - but these
symptoms often creep in slowly, and are easy to miss unless you understand the
underlying physiology.
In the last post, we explored what happens when thyroid hormone levels are too high — and how to reason through the possible causes using the HPT axis. Now, we turn our focus to the three most common conditions that medical students are likely to encounter in practice: Graves’ disease, toxic multinodular goitre (TMNG), and thyroiditis.
These conditions may all present with elevated T3 and T4,
but they arise from fundamentally different mechanisms (autoimmune
stimulation, nodular autonomy, and inflammatory leakage) and require very
different approaches to diagnosis and management.
In this post, we’ll walk through each condition in detail,
exploring the pathophysiology, clinical features, investigations, and reasoning
that help distinguish them. By the end, you’ll be able to interpret thyroid
function tests, understand scan results in context, and explain
to patients why their thyroid is misbehaving !
What happens when there’s too much thyroid hormone in the body?
When thyroid hormone levels are elevated, the body doesn’t
just “speed up”, it becomes overstimulated across multiple systems. This isn’t
a random collection of symptoms; it’s a predictable physiological response to
excess T3 and T4.
The thyroid axis is one of the most common clinically encountered endocrine systems. It regulates metabolism, temperature, energy, and mood. Understanding how it works helps you reason through symptoms like fatigue, weight change, palpitations, constipation, and menstrual irregularities.
Welcome to endocrine! This post is designed to help you reason through the hypothalamic–pituitary axis (HPA), not just memorise it. You don’t need to know everything yet. What matters is understanding how the system works, how it regulates itself, and how that explains common clinical presentations.
Exams are just under eight weeks away - close enough to feel real, but still enough time to prepare steadily without panic. The key now is consistency. Small, regular efforts will carry you much further than bursts of late-night cramming.
